Change shebang to use python3.
Handle missing colorsysplus module gracefully.
Config parsing: Allow homedir relative sound path.

This change allows this kind of sound specification:

[Sounds]
blip = ~/media/message.ogg
Find config file in user specific config location.

The previous code tried to look in $XDG_CONFIG_HOME, however this was
neither expanded, nor does this variable have to be set according to the
XDG spec. Using the appdirs module, we can find the correct place easily.

l