Adding vlc plugin for firefox.

I really wanted it to substitute the html5 player, or at least the direct
player for mp4/webm files in firefox, but I couldn't make it work. The
formats recognized by the firefox internal player were used in all cases. The
plugin worked for formats unknown by firefox.

https://support.mozilla.org/ca/questions/1089501

Nevertheless, as I wrote the nix recipe, I commit it. It may be of interest to
someone else.