Add the config env command (fixes #620)

[snoyberg]

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

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• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

New integration test added.

[sjakobi]

@snoyberg This looks useful! :)

I'm wondering though whether the stack config namespace is the right place for this functionality:

≻ stack exec -- stack config --help
Usage: stack config COMMAND [--help]
  Subcommands specific to modifying stack.yaml files

Available options:
  --help                   Show this help text

Available commands:
  set                      Sets a field in the project's stack.yaml to value
  env                      Print environment variables for use in a shell

Run 'stack --help' for global options that apply to all subcommands.

But it might be enough to change the line Subcommands specific to modifying stack.yaml files.

[snoyberg]

I was also unsure of using config, I'm glad you raised it. My decision came from the fact that the original issue mentioned that namespace from other tooling, so it seemed consistent. I'm happy to either give this a different name or fix the help text. If I don't hear from others, I'll go the direction of changing the help text.