Add support for rubocop formatter

[zinovyev]

This pull request adds a rubocop support for Ruby and ERuby file types.
Solves #30

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[zinovyev]

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[dbarnett]

Could you see about adding a vroom test file with examples of rubocop's expected input/outputs, like other formatters have under the vroom/ directory?

[dbarnett]

This formatted[1:] is a little strange… does the formatter output an extra first line to stdout that's not part of the output?

[akveritas]

Is anyone still working on this? I was about to start doing this when I found it already done and pending review.

[fowles]

Is anyone still working on this? I was about to start doing this when I found it already done and pending review.

I suspect that David was waiting for a response to his question. If you want to take over and send a new PR that is reasonable