app.demux: implement "exclusive" demultiplexing

This goes towards #88 but still needs testing and documentation.
app.demux: add verbosity
app.demux: fix specification parsing
app.demux: fix filter for Undetermined libraries

This should be the last step of #88.
Merge branch 'exclusive_demux' into v0.3-dev
app.demux: add option gzip-buffer-size-factor