Validate configuration (issue #8)

Added Joi schema to validate configuration objects. Solves #8.
Are you sure this raises the error effectively? The task raises errors through the event emitter. If simply throwing a new PluginError works, great. If not, let's report the error in the leading parameter of a callback (again, outside of the parser/processor), and handle the error with standard node callback conventions where we can leverage the event emitter on ```this```.