Linter for Elixir: credo

[coala-bot]

See https://github.com/rrrene/credo

Opened via gitter by @nemaniarjun

[jayvdb]

@robinsjdotcom are you planning to continue working on this? pleaseeee! ;-)

[palash25]

Is someone working on this? I see a closed PR.

[Makman2]

Doesn't look like ;)

[sangamcse]

Please make the spelling of Elixir correct so that it will be easy to search open issues of that name.
And I feel that the difficulty should be high because before adding credo, it will need Mix dependency to add.