vimrc: source the built-in matchit plugin

Allows for extended % matching, between things like < & >, HTML opening
and closing tags, and general regex for various languages.
Add ftplugin/{html.vim,htmldjango.vim}

Gives me matchit plugin settings for HTML and Django templates without
including the entire ftplugin for either. Copied the settings from vim's
default ftplugins. Allows me to use matchit to jump between HTML tags
and <:>.