Add a mode for omxplayer local sound playback

Add a "--player omxplayerlocal" option, that plays audio through the local
output instead of the hdmi output with omxplayer.
Configure yt default player using a ~/.config/yt settings file

The user can set the default player in a ~/.config/yt settings file, by
reading the "player" entry, in the "[General]" section of the settings
file. The format of the settings file should be like this:

[General]
player = omxplayer

If the file is not there, or if it's corrupted in any way, yt defaults
to mplayer.
Update README.rst with info about the settings file.

Add information in the README.rst file about creating the ~/.config/yt
settings file.