TODO:
* Add GStreamer pipeline builder
* Provide CLI interface

NOTES:
Mixing two sources together using GStreamer "adder" element:
gst-launch adder name=mix ! pulsesink { filesrc location=05.mp3 ! decodebin ! mix. } { filesrc location=07.mp3 ! decodebin ! mix. }

What I'm doing in this GStreamer pipeline:
gst-launch adder name=mix ! vorbisenc ! oggmux ! filesink location=blah { pulsesrc ! mix. } { pulsesrc ! mix. }