Add doctor command

[kkirsche]

Redo of #137 because squashing broke the PR

[kkirsche]

 ~/git/go/src/github.com/twpayne/chezmoi >> cmd/doctor $ go build -o cmoi
 ~/git/go/src/github.com/twpayne/chezmoi >> cmd/doctor $ ./cmoi doctor
Arch:                    amd64
OS:                      darwin
Configuration File:      Exists      (-rw-r--r-- /Users/kkirsche/.config/chezmoi/chezmoi.yaml)
Destination Directory:   Exists      (-rwxr-xr-x /Users/kkirsche)
Source Directory:        Exists      (-rwx------ /Users/kkirsche/.local/share/chezmoi)
Bitwarden:               Not Found   (looking for: bw)
Editor:                  Installed   (-r-xr-xr-x /usr/local/bin/nvim)
LastPass:                Not Found   (looking for: lpass)
Pass:                    Not Found   (looking for: pass)
Vault:                   Installed   (-r-xr-xr-x /usr/local/bin/vault)
VCS:                     Installed   (-rwxr-xr-x /usr/local/bin/git)

[twpayne]

Many thanks for the PR and your patience with the review comments.

[kkirsche]

Of course. It's important to try to get this stuff as correct as possible at the first go, and it helps me learn or consider different approaches to a problem than I would have taken. Appreciate the feedback and opportunity to contribute