DDLSHELL-25. Add parser plugin SPI.

Adds ParserPlugin and ParserPluginFactory SPIs.
Other framework components can now extend the Kiji schema shell
with new syntax.

* Use SHOW MODULES; to show available and loaded modules.
* Use MODULE <m>; to load a module named <m>.
* Adds test kit for module SPI compliance (ParserPluginTestKit).
* Environment now keeps a list of loaded modules.
* Adds tests for module SPI system.