This is based largely on manveru's example implementation:
http://paste.linuxhelp.tv/pastes/view/15309
NOTE: we should plan on ripping this out once nested params
makes it into Rack.
Adds an "provides"-option to a route definition, which can either be
a string specifying a MIME type, a symbol that Rack can resolve to
a MIME type, or an array of either.