[#835] New cop BarePercentLiterals

The style guide says:
Use %() (it's a shorthand for %Q) for single-line strings
which require both interpolation and embedded double-quotes.

This cop's job is to check that you use % rather than %Q,
or the opposite if so configured.
Merge pull request #1209 from jonas054/835_bare_percent_literal

[#835] New cop BarePercentLiterals