Add Ruby script linter

Add a linter that extracts code from a HAML document using the
`ScriptExtractor` and runs it against `rubocop`, a well-developed Ruby
linter.

This provides really powerful linting for `haml-lint` right out of the
box, as most of the common lints (e.g. variable assigned but not used,
single instead of double quotes, etc.) are handled by Rubocop.

The only trick is converting the lint lines reported by Rubocop back to
the lines of the original HAML document. This is achieved by using the
source map of the `ScriptExtractor`, as it keeps track of which lines of
the original HAML document the extracted code came from.