Lack of a formatters interface

[Drowze]

Hello @grodowski 👋

First of all, I'd like to say thank you for this great project! I've been using it extensively for the past year on the CI of a few internal repositories and it's quite handy to me and my peers. Great work!

After some time with undercover however, I feel that it lacks a formatter interface. I see on this line a comment that even hints that a formatter interface was planned: https://github.com/grodowski/undercover/blob/master/lib/undercover/result.rb#L63

    # TODO: create a formatter interface instead and add some tests.

That said, this is something that concerns me so I'd like to help 😄 I've been actually working a bit on some changes (which you can check here) that aim de-couple the Result from Formatter - thus allowing other formatters to be used.
I would personally like to see this being possible (which I think is exactly how rubocop handle this):

undercover --require $(pwd)/my_custom_formatter.rb --formatter MyCustomFormatter

# or if the formatter is on a gem and the gem is on the gemfile along with undercover:
bundle exec undercover --formtter MyCustomFormatter

I'm a bit concerned though as this would be a slightly big change - so I'd like to ask your opinion before opening my first PR on this project. What do you think @grodowski? Can I go ahead and try to tackle this one? 😄

[grodowski]

Hey @Drowze! Thank you for the kind words and your willingness to contribute to the project! This formatter TODO is really old and I couldn't find a good reason to get back to it and implement a formatter class.

I went through your changes and think there are two aspects: one being about decoupling Result and Formatter and the other to provide a formatter argument to the CLI.

I have been usually suggesting pronto and pronto-undercover which provides a --formatter option and comes with a suite of handy ones like github_pr already. So what are the benefits of adding a pretty similar interface to undercover itself? Do you have a use case in mind for when this new option would become available?