libav: tweak formula to prevent unwatned behavior

The old formula would build with flags the user didn't necessarily
ask for which could lead to nastiness, e.g. if you happen to have lame
installed then libav would be compiled with support for lame. If one
later uninstalled lame libav would break. Further more, the install
reciept wouldn't reflect the actual build options used leaving homebrew
unable to inform the user of the missing dependancy.

I tweaked the formula to only build with certain codecs if the user
explicitly says to do so. Additionally I added some 'popular' codecs to
be built in by defualt.