bug#25022: 25.1.50; Different highlighting for different citation level in message-mode

[Hong Xu]

Currently in message-mode all cited texts are highlighted in the same
way. It would be nicer if the highlighting of different citation levels
can be easily customized.

In GNU Emacs 25.1.50.13 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.5)
 of 2016-11-24 built on home
Repository revision: 7f43d7c0101256b120ead3ddae6ee47327d231c9
Windowing system distributor 'The X.Org Foundation', version 11.0.11604000
System Description:     Debian GNU/Linux 8.6 (jessie)

Configured using:
 'configure --with-modules --with-xwidgets
 --prefix=/home/hong/.local/opt/emacs/
 '--program-transform-name=s/^ctags$/ctags.emacs/''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 MODULES XWIDGETS