Add different faces for different citation levels in Message mode

Add different faces for different citation levels in Message mode

* message.el (message-font-lock-keywords)
(message-font-lock-make-cited-text-matcher): Add support for
different faces for different citation levels.  The faces are
defined in the faces named `message-cited-text-N': N of the
Mth citation level will be M mod 4.
(message-cited-text-1, message-cited-text-2)
(message-cited-text-3, message-cited-text-4): Add customization
for the faces of 4 different citation level.  In the future, the
number of faces may increase, as the code is flexible enough to
automatically deal with that.
(message-cite-level-function): Add a function to customize the
determination of cite levels given the prefix of the cited text
(bug#25022).