* riece-ctcp.el: Require 'riece-highlight.

(riece-ctcp-action-face): New face.
(riece-ctcp-action-face): New variable.
(riece-ctcp-action-prefix): New variable.
(riece-ctcp-requires): Require 'riece-highlight if it is enabled.
(riece-ctcp-insinuate): Modify riece-dialogue-font-lock-keywords
if 'riece-highlight is enabled.
(riece-handle-ctcp-action-request): Prepend
riece-ctcp-action-prefix instead of riece-change-prefix.
(riece-command-ctcp-action): Ditto.