feat: Adds variable to use custom config file with Elixir Credo

[Thau]

Reopened version of #3331 because stale-bot closed it, including @w0rp's suggestions.

Adds g:ale_elixir_credo_config_file to allow users to use a custom configuration file for the checks. Motivation is my company using a centralised configuration file for all of our Elixir projects, so it's our belief that more people could benefit from this option.

Thank you!

[stale]

This pull request has been automatically marked as stale because it has not been updated recently. Make sure to write tests and document your changes. See :help ale-dev for information on writing tests.
If your pull request is good to merge, bother w0rp or another maintainer again, and get them to merge it.

[Thau]

@w0rp ?

[stale]

This pull request has been automatically marked as stale because it has not been updated recently. Make sure to write tests and document your changes. See :help ale-dev for information on writing tests.
If your pull request is good to merge, bother w0rp or another maintainer again, and get them to merge it.

[Thau]

Still ready to merge, stalebot :P

[hsanson]

Thanks for waiting... looks good to me.

[Thau]

@hsanson, no worries, I understand you guys have quite some PRs to watch for. It's been a pleasure!