Beautiful integration

Default color values can be overridden by theme values:

theme.apw_fg_color
theme.apw_bg_color
theme.apw_mute_fg_color
theme.apw_mute_bg_color
Mixer support

Mixer command ("pavucontrol" by default) is executed when
widget is right-clicked.

Mixer command can be customized by calling :SetMixer() on the widget.
Update documentation for mixer and theme support

Updated README.md to reflect new mixer and theme features.
Mixer fixes

Changed mixer command to "LaunchMixer()"
Corrected spaces/tabs issues.
Documentation fixes

Removed hanging semicolon in code snippet.
Added note about the need to run beautiful.init before requiring apw.