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
```