Call instance method on email processor

Rather than sending Griddler.configuration.processor_method to
Griddler.configuration.processor_class with a Griddler::Email as an
argument (i.e., EmailProcessor.process(email)), instantiate
Griddler.configuration.processor_class with a Griddler::Email and call
Griddler.configuration.processor_method on that object (i.e.,
EmailProcessor.new(email).process).

This encourages the behavior of separating logic into methods within an
EmailProcessor, which is better than having a single class method with
many responsibilities or passing state around between class methods.
Might want to change this doc string and the one below.