Use minitest 5.
Move to a system of transcoders and stronger GIGO configuration.

* Document all changes in README.
* New GIGO.encoding getter/setter.
* New GIGO.transcoders list. Users can tune mix to their will.
* GIGO.load now takes an options arg which can control transcoders on
  an individual load/transcode call vs altering all transcoders.
* Test all the new hotness.