Hook for modifying a citation just inserted in the mail buffer.
Each hook function can find the citation between (point) and (mark t),
and should leave point and mark around the citation text as modified.
The hook functions can find the header of the cited message
in the variable `mail-citation-header', whether or not this is included
in the cited portion of the message.
If this hook is entirely empty (nil), a default action is taken instead of no action.