plugins.multiplexer: rename filter keywords

Rename them from filter_out and filter_only to filter-out and filter-only,
respectively.

Motivation: consistent with original RFC and easier to the eyes, specially when
new filter keywords get added (filter-depth, filter-value, etc).
examples: add a relatively complex multiplexer file

This is the tree we've been using in several of our discussions and
algorithm simulations.
Merge pull request #179 from adereis/mux

multiplexer misc commits