Use pausing_keep_force to stop MPlayer resuming playback while paused

`pausing_keep_force` was introduced in 2008 (r27665), and while still
marked as experimental, should be stable for getting and setting simple
properties.

`pausing_keep` causes MPlayer to advance one frame every time a command
is issued.

Fixes issue #11.
Check for MPlayer support of pausing_keep_force (r27665)

`pausing_keep_force` was introduced in r27665, and was released in
version 1.0rc3.
Pre-compile regular expression, streamline match checking
Ensure MPlayer never shows a video output

Be overly paranoid with MPlayer arguments.
Move call to MPlayer.terminate before raising exception
Add note in README about MPlayer minimum version