Add --format option

[andersonkrs]

Hey guys!
At my work, we adopted this gem and we are trying to integrate it with this guy: https://github.com/reviewdog/reviewdog in our CI. But, it has been really hard to do, because this tool uses vim's errorformat and it's not that easy to parse the actual output.

I've seen that there are two proposals to tackle this problem: #84 #116, and it hasn't been any movement, so I decided to do this starting point.
This implementation that I've made is strongly inspired by these PRs.

I've added a --format option, with that, users can pass the desired output format. To add a new format is straightforward, you just need to create a new inherited class from Reporter and implement the method show and it will be available on cli automatically.

So, I've refactored and extracted all the code responsible for reporting results in 2 new classes:
1 - MultilineReporter - The default report format
2 - CompactReporter - Oneliner report, outputs data using format: file:line:column: message. This guy will be really useful for many kinds of integrations like CI, editors, etc. 😃

This opens the way to include new output formats more easily, like JSON, XML, etc.

[fidalgo]

I'm looking into this to configure ale and CI to have a consistent style across all projects.

[rafaelfranca]

Looks good. Can you document this new option in the README?

[andersonkrs]

@rafaelfranca Done.

I did one more little thing, I've extracted the responsibility of show preview messages like Linting x files ... into reporter object, because other formats like JSON, XML, etc, can't have that on output.

[ni3t]

Hello,

Is there any progress on merging this change? We are migrating our CI to github actions, and the ability to write a custom reporter would make annotating errors in the changed files view a lot easier. No rush if this is still a WIP.

[brendo]

With the newly released 0.0.36, this PR seems to introduce an error:

$ bundle exec erblint --lint-all
NoMethodError: undefined method `delegate' for ERBLint::Reporter:Class
Did you mean?  DelegateClass
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint/reporter.rb:36:in `<class:Reporter>'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint/reporter.rb:5:in `<module:ERBLint>'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint/reporter.rb:4:in `<top (required)>'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint.rb:16:in `require'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint.rb:16:in `<top (required)>'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint/cli.rb:3:in `require'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint/cli.rb:3:in `<top (required)>'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/exe/erblint:6:in `require'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/exe/erblint:6:in `<top (required)>'
  /tmp/bundle/ruby/2.6.0/bin/erblint:23:in `load'
  /tmp/bundle/ruby/2.6.0/bin/erblint:23:in `<top (required)>'

[andersonkrs]

With the newly released 0.0.36, this PR seems to introduce an error:

$ bundle exec erblint --lint-all
NoMethodError: undefined method `delegate' for ERBLint::Reporter:Class
Did you mean?  DelegateClass
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint/reporter.rb:36:in `<class:Reporter>'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint/reporter.rb:5:in `<module:ERBLint>'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint/reporter.rb:4:in `<top (required)>'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint.rb:16:in `require'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint.rb:16:in `<top (required)>'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint/cli.rb:3:in `require'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/lib/erb_lint/cli.rb:3:in `<top (required)>'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/exe/erblint:6:in `require'
  /tmp/bundle/ruby/2.6.0/gems/erb_lint-0.0.36/exe/erblint:6:in `<top (required)>'
  /tmp/bundle/ruby/2.6.0/bin/erblint:23:in `load'
  /tmp/bundle/ruby/2.6.0/bin/erblint:23:in `<top (required)>'

Oh boy, I'll check this out soon