Support for language-agnostic documentation

[pyhedgehog]

I'm writing docs with i18n support.
There are two places where it should be done:

1. All docstrings of click-command functions and options help arguments. Like this:

@click.option('--quiet/--verbose', '-q/-v', flag_value=True, default=False, help=_("Disable processing output"))
@doc(_("""Help text of subcommand."""))
def subcommand(quiet):
    ...

2. MkDocs docs directory. I'm using mkdocs-static-i18n plugin. So I've created index.XX.md for each supported language.

I want to pass different language code to mkdocs-click plugin in each cli.XX.md.
I think it will includes reloading of module each time with changing LANG variable.

[pyhedgehog]

@DataDog, should I try to implement it and create pull request or you want to do it alone?

[ofek]

Feel free to open a PR!