Allow user to integrate custom tuners

[micahjsmith]

Autobazaar currently supports three tuners: gp, gpei, and uniform. It allows users to choose from among them. It is not currently supported to use a different tuner, even if the tuner follows the BTB Tuner API.

Suggested behavior:

• modify autobazaar.search.PipelineSearcher to take a more flexible input tuner_type
• if tuner_type is a string, look up a tuner class in btb.tuning.tuners by matching on case-insensitive names with or without Tuner suffix.
• if tuner_type is a string with the FQ name of a class, then try to import the tuner, confirm it is a subclass of BaseTuner, and then proceed.
• else if tuner_type is a subclass of btb.tuning.tuners.base.BaseTuner then proceed and instantiate an instance of this tuner
• else error