Show error messages from the pipeline

Fixes #8
Fix application lock-up when stopping http streams.

Fixes #9
Force the pipe to stereo

This implements a capsfilter equivalent to

   $ gst-launch .. ! audioconvert ! audio/x-raw,channels=2 ! ..

to force the pipe to two channels. This way, mono files will
be played to both channels.

Fixes #10, #11
Allow timelabel colours to be set from the commandline

Instead of hardcoding the background colour into 4deckradio, allow the
user to provide them on the command line:

    $ 4deckradio -r "#ff7070" -g "#D0F0E4" -y "yellow"

Fixes #14
Update README.md to include new timelabel colour options