Ability to ignore warnings of certain types

[TSMMark]

Wondering if it would be possible to add a rubocop.yml config option to ignore or exclude certain warning types?

Jeremy Evans has this gem called warning: https://rubygems.org/gems/warning which appears to have a few nice features, mainly:

ability to ignore specific warning messages

e.g. Warning.ignore(:missing_ivar)

Maybe this gem could introduce some similar option, either copying some of the code from warning gem, or adding warning gem as a dep of rubocop-rubycw. warning gem does not appear to have any dependencies so it should be a pretty light addition.

Thoughts?