Merge branch 'bug/242' into 'master'

Handle a previously unhandled code scenario

Previously, we didn't handle the case where an error code was implicitly
ignored (by not being in --select) and implicitly selected (by not being
in --ignore). This means we need to update StyleGuide#_decision_for and
StyleGuide#is_user_selected to handle these cases.

Closes #242

Related-to #239

Related-to !132

See merge request !133