Fix --msglevel for new mpv release
players/mplayer: Unset TERM to avoid escape sequences

At least mpv may output escape sequences which result in syncplay trying
to parse something like "\x1b[?1l\x1b>ANS_filename=blah.mkv". Work
around this by unsetting TERM.
Merge pull request #32 from ion1/master

Fix compatibility with the 0.3.x mpv release