Hydra example SearchPath plugin

This plugin provides an example for how to write a SearchPathPlugin that can manipulate the search path. Typical use cases includes:

A framework that wants to allow user code to discover its configurations and be able to compose with them.
A plugin that wants to extend Hydra or another plugin by providing additional configs in existing config groups like hydra/launcher.
A plugin that can replace the default configuration of another plugin or of Hydra itself by prepending its configurations before those that it want to replace.

SearchPath plugins are discovered and enabled automatically once they are installed. You can use python foo.py hydra.verbose=true --cfg to see the search path and the installed plugins.