Add js-beautify support to JS generator

[wing328]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.3.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

To turn on the feature, please setup the environment variabel: export JS_BEAUTIFY_PATH=/usr/local/bin/js-beautify

e.g.

[main] INFO  o.o.codegen.AbstractGenerator - writing file /Users/williamcheng/Code/openapi-generator/samples/client/petstore/javascript/docs/PetApi.md
[main] INFO  o.o.codegen.AbstractGenerator - writing file /Users/williamcheng/Code/openapi-generator/samples/client/petstore/javascript/src/api/StoreApi.js
[main] INFO  o.o.c.l.JavascriptClientCodegen - Successfully executed: /usr/local/bin/js-beautify -r -f /Users/williamcheng/Code/openapi-generator/samples/client/petstore/javascript/src/api/StoreApi.js

cc @CodeNinjai (2017/07) @frol (2017/07) @cliffano (2017/07)

[cliffano]

LGTM, but just curious, does the CI build actually run and test the generated petstore? (beyond generating the code)
Wondering if there's any, however unlikely, possibility of beautification causing any unexpected issue.

[wing328]

@cliffano yes, there are integration tests, e.g. https://github.com/OpenAPITools/openapi-generator/blob/master/samples/client/petstore/javascript/test/api/PetApi.spec.js

[wing328]

BREAKING CHANGE: we'll later rename the environment variable to JS_POST_PROCESS_FILE to make it consistent with other generators.