Allows for generating multiple outputs via config. Just specify multiple
config files on command line.

Intent for this is to reduce CI times to generate outputs as well as to
reduce time for users to run ensure-up-to-date to meet PR standards.

Example command:

  openapi-generator batch --includes-base-dir `pwd` --fail-fast  -- bin/ci/*

---

As part of this implementation, the batch command support a customized
JSON key, `!include`. If this key's value refers to an existing file,
that file's contents are "unwrapped" into the config during
deserialization. This allows us to easily point to the same configs used
by our sample scripts without modifying the CLI generate task's switches
or assumptions.

* master: (110 commits)
  [golang] Regenerate all go samples  (#3988)
  Better tests for string (number) (#3953)
  Add missing enum processing in C++ codegen, already present for Qt5 (#3986)
  [C++] [Pistache] Removed deprecated warnings (#3985)
  [C++][Pistache] Simplified model template (#3417)
  add go oas3 petstore to ensure up-to-date (#3979)
  replace gitter with slack in the doc (#3977)
  Fix wrong variable name in LessThan and LessThanOrEqual asserts (#3971)
  #3957 - Removed hardcoded baseUrl (#3964)
  Regenerate go openapi3 samples (#3975)
  [rust] Make it easier to test rust client generator (#3543)
  Fix issue3635 (#3948)
  add gradle repository (#3867)
  [java] allow to use setArtifactVersion() programmatically (#3907)
  Add a link to DevRelCon SF 2019 (#3961)
  Add a link to a medium blog post (#3960)
  update maven-compiler-plugin version (#3956)
  fix generateAliasAsModels in default generator (#3951)
  Implement BigDecimal to Decimal in swift4 for currency data as type=string format=number (#3910)
  Add F# Functions server generator (#3933)
  ...

* master: (35 commits)
  [haskell-http-client] update samples (#4073)
  [haskell-http-client] Bump deps to LTS 14.7 (#4068)
  update release for 4.2.0
  [typescript-axios] Fix api generating incorrect seralization type check (#4051)
  prepare 4.1.3 release (#4052)
  typescript-node: form data file (#3967)
  Add a link to blog post on vertx and openapi (#4048)
  better wording for apiNameSuffix option description (#4045)
  [Ruby] fix ruby test, update error message (#4041)
  [PHP] Correctly format JSON in headers (#4024)
  [haskell-http-client] add dateTimeParseFormat cli option - overrides the format string used to parse a datetime (#4037)
  Add frankyjuang to the C# technical committee (#4036)
  Feature/api name suffix (#3918)
  [F#] minor improvements to the generators (#3968)
  Repaired Checkstyle (#4029)
  mockito 3.1.0 (#4035)
  typescript-fetch: fix return type of primitive value (#4028)
  [typescript][node]: Add accept header if produces is not empty (#3966)
  [haskell-http-client] disable unused import warning in Core.hs (#4020)
  Add a link to the tutorial in http4k (#4019)
  ...

…efore merge