Transfer responsibility of offence format and color to formatters

Also made severity coloring logic DRY.
Remove duplicated #report_summary specs

ClangStyleFormatter just uses inherited #report_summary.
Add Offence#corrected?
Print [Corrected] if the offence is auto-corrected in some formatters

This affects to simple/clang/progress/emacs formatters.
Add `corrected` value to offence objects in JSON formatter
Print correction count in final summary if corrected any offences

This affects to simple/clang/progress formatters.
Merge pull request #509 from yujinakayama/indicate-autocorrection

Indicate corrected offences in formatters