Replace syntax/mkd.vim with vim-markdown.

This also includes incorporating some of vim-markdown's setting into our
SetupMarkdown(), and adding a little extra highlighting for code blocks.
Make the default filetype for Markdown files be "markdown".

It appears "markdown" is a more common filetype for markdown files, so
it seems to be a better default.  To provide some support for mkd, just
in case, mkd snippets now delegates to markdown.

This change also means that anyone with custom mkd snippets and
autocommands needs to change them to markdown instead.

Also, we now use vim-markdown's filetype detection.  Now .mkdn is
recognized as a markdown filetype and now .mdwn is not.  This seems
reasonable since .mdwn seems unlikely to be used.
user/jszakmeister: move markdown snippets to the new filetype
Implement better embedded language support for vim-markdown.
Support embeddeding sh and vim in rst and markdown documents.
user/jszakmeister: enable support for trac-style code blocks in markdown