Add support for Pronto Ruby linter

[alexmclean]

Is your feature request related to a problem? Please describe.
It appears that super-linter does not support linting only lines that have changed and has no plans to per this issue: #219

But this is something that we have really found useful working with a large legacy codebase that we can fix incrementally, and it is fully supported with Rubocop if you use Pronto: https://github.com/prontolabs/pronto

Describe the solution you'd like
Would you consider supporting Pronto within super-linter? My understanding is that it can be used for running Rubocop or Flay, but Rubocop is the one that is already supported by super-linter.

[zkoppert]

Yeah, that sounds reasonable to me that we could support linting only diffed lines in in Ruby. I imagine that would need to be a configuration option though.

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