Add `cc' support.

Add `cc' support.
This was reported by espie@ and in the TODO.
Caveat: `cc' has buggy behaviour when invoked in groff(1) and followed
by a line-breaking control character macro, e.g., in a -man doc,

  .cc |
  .B foo
  'B foo
  |cc
  'B foo

will cause groff(1) to behave properly for `.B' but inline the macro
definition for `B' when invoked with the line-breaking macro.