Support markdown mode

My markdown-mode.el copy is in ~/.emacs.d/lisp. The original comes from

	git://jblevins.org/git/markdown-mode.git

The version in Emacs 24 from Ubuntu is part of goodies.el.  As anything else
in Ubuntu's Emacs, it is hopelessly behind.

Both *.md and *.markdown are recognized as markdown.

modified:   .emacs.d/init.el