Add a CopCount formatter

Allows you to see a list of cops with offences, ordered by offence count.

e.g.
```
❯ be rubocop -f p -f copcount app/controllers
Inspecting 98 files
CCCCCCCCCCCCCCCWCCCCC..CCCCCC.CCCCCCCC.CCWWCCCCCC.CCCCW..CCCCCWCCCCCCCCCWCCCC.CWCCCC.CCCCCCCCWCCCC

Offences: ...SNIP...

98 files inspected, 134 offences detected

(87)	Documentation
(12)	DotPosition
(8)	AvoidGlobalVars
(7)	EmptyLines
(6)	AssignmentInCondition
(4)	Blocks
(4)	CommentAnnotation
(3)	BlockAlignment
(1)	IndentationWidth
(1)	AvoidPerlBackrefs
(1)	ColonMethodCall
```