term: Fix confirmation prompts on windows

[nlowe]

This fixes confirmation prompts on windows by scanning for a line of input and comparing that to the desired prompt. term.Confirm(...) is refactored slightly to make it easier to test.

Fixes #347

Note that other tests (not those added in this PR) will fail on windows until #411 is merged.

I've also verified this works on windows:

With `tk` `v0.12.0`

C:\Users\Nathan\projects\grafana-agent-operator\example [master ≡ +0 ~4 -0 !]> tk apply .\environments\default\
diff -u -N C:\Users\Nathan\AppData\Local\Temp\LIVE-097035773/apiextensions.k8s.io.v1.CustomResourceDefinition..servicemonitors.monitoring.coreos.com C:\Users\Nathan\AppData\Local\Temp\MERGED-483356984/apiextensions.k8s.io.v1.CustomResourceDefinition..servicemonitors.monitoring.coreos.com
--- C:\Users\Nathan\AppData\Local\Temp\LIVE-097035773/apiextensions.k8s.io.v1.CustomResourceDefinition..servicemonitors.monitoring.coreos.com       2020-10-27 22:12:54.118744100 -0700
+++ C:\Users\Nathan\AppData\Local\Temp\MERGED-483356984/apiextensions.k8s.io.v1.CustomResourceDefinition..servicemonitors.monitoring.coreos.com     2020-10-27 22:12:54.142744000 -0700
@@ -46,7 +46,7 @@
         f:storedVersions: {}
     manager: kubectl-client-side-apply
     operation: Update
-    time: "2020-10-28T05:04:03Z"
+    time: "2020-10-28T05:12:51Z"
   name: servicemonitors.monitoring.coreos.com
   resourceVersion: "545577"
   selfLink: /apis/apiextensions.k8s.io/v1/customresourcedefinitions/servicemonitors.monitoring.coreos.com

Applying to namespace 'default' of cluster 'docker-desktop' at 'https://kubernetes.docker.internal:6443' using context 'docker-desktop'.
Please type 'yes' to confirm: yes
aborted by user

With `tk` built from this commit

C:\Users\Nathan\projects\grafana-agent-operator\example [master ≡ +0 ~4 -0 !]> ..\..\tanka\tk.exe apply .\environments\default\
diff -u -N C:\Users\Nathan\AppData\Local\Temp\LIVE-142144125/apiextensions.k8s.io.v1.CustomResourceDefinition..servicemonitors.monitoring.coreos.com C:\Users\Nathan\AppData\Local\Temp\MERGED-139668408/apiextensions.k8s.io.v1.CustomResourceDefinition..servicemonitors.monitoring.coreos.com
--- C:\Users\Nathan\AppData\Local\Temp\LIVE-142144125/apiextensions.k8s.io.v1.CustomResourceDefinition..servicemonitors.monitoring.coreos.com       2020-10-27 22:04:00.340253300 -0700
+++ C:\Users\Nathan\AppData\Local\Temp\MERGED-139668408/apiextensions.k8s.io.v1.CustomResourceDefinition..servicemonitors.monitoring.coreos.com     2020-10-27 22:04:00.366253100 -0700
@@ -46,7 +46,7 @@
         f:storedVersions: {}
     manager: kubectl-client-side-apply
     operation: Update
-    time: "2020-10-28T05:03:23Z"
+    time: "2020-10-28T05:03:57Z"
   name: servicemonitors.monitoring.coreos.com
   resourceVersion: "545509"
   selfLink: /apis/apiextensions.k8s.io/v1/customresourcedefinitions/servicemonitors.monitoring.coreos.com

Applying to namespace 'default' of cluster 'docker-desktop' at 'https://kubernetes.docker.internal:6443' using context 'docker-desktop'.
Please type 'yes' to confirm: yes
namespace/etcd unchanged
namespace/foo-a unchanged
namespace/foo-b unchanged
namespace/foo-c unchanged
namespace/monitoring unchanged
serviceaccount/grafana-agent-cluster unchanged
configmap/cortex-config unchanged
configmap/dashboards-0 unchanged
configmap/dashboards-1 unchanged
configmap/dashboards-2 unchanged
configmap/dashboards-3 unchanged
configmap/dashboards-4 unchanged
configmap/dashboards-5 unchanged
configmap/dashboards-6 unchanged
configmap/dashboards-7 unchanged
configmap/grafana-agent-cluster unchanged
configmap/grafana-config unchanged
configmap/grafana-dashboard-provisioning unchanged
configmap/grafana-datasources unchanged
persistentvolumeclaim/cortex-data unchanged
customresourcedefinition.apiextensions.k8s.io/servicemonitors.monitoring.coreos.com configured
clusterrole.rbac.authorization.k8s.io/grafana-agent-cluster unchanged
clusterrolebinding.rbac.authorization.k8s.io/grafana-agent-cluster unchanged
service/etcd unchanged
service/example unchanged
service/example unchanged
service/example unchanged
service/cortex unchanged
service/grafana unchanged
service/grafana-agent-cluster unchanged
deployment.apps/etcd unchanged
deployment.apps/example unchanged
deployment.apps/load-generator unchanged
deployment.apps/example unchanged
deployment.apps/load-generator unchanged
deployment.apps/example unchanged
deployment.apps/load-generator unchanged
deployment.apps/cortex unchanged
deployment.apps/grafana unchanged
deployment.apps/grafana-agent-cluster configured
servicemonitor.monitoring.coreos.com/example unchanged
servicemonitor.monitoring.coreos.com/example unchanged
servicemonitor.monitoring.coreos.com/example unchanged
servicemonitor.monitoring.coreos.com/cortex unchanged

[sh0rez]

Nice work! Sorry it took so long to review. And thanks for adding a test here :)