quvi: Add support for streaming sites using libquvi

quvi: Add support for streaming sites using libquvi

This enables playing URLs from libquvi supported streaming sites
directly, e.g. "mplayer http://www.youtube.com/watch?v=...."

Anything opened with mplayer is checked with libquvi. If it looks like
a URL of a supported streaming site, libquvi is used to extract the
media URL, which is then passed to the lower level mplayer code
instead of the HTML URL. Hopefully the libquvi URL checker works well
enough that it doesn't cause any problems with normal URLs, files, or
whatever else mplayer's stream layer accepts.

Add the --quvi-format option. The option value is directly passed
to libquvi as requested format. The only values that seem to work for
any streaming site seem to be "best" (best quality) and "default"
(lowest quality). The mplayer option defaults to "best" (overriding
libquvi's default).

Outstanding issues:
- Does libquvi checking every opened file really not cause problems?
  Should there be a runtime option to disable libquvi use?
  (Probably not an issue.)
- Should we check/set the supported protocol? By default libquvi has
  support for all protocols enabled. In the worst case, it might
  return an URL using a protocol not supported by mplayer, even though
  it could extract URLs with other protocols too.
  (Probably not an issue.)
- Somehow export metadata (like media title) to the mplayer frontend?