Expose hook for creating handlers

[sloria]

This is independently useful of #67 , but it would meet that use case, since one could do

class MyCustomMllpRequestHandler(mllp._MLLPRequestHandler):
    def _create_handler(self, *args, **kwargs):
        handler = super()._create_handler(*args, **kwargs)
        handler.server = self
        # ... set more attributes
        return handler

[coveralls]

Coverage increased (+0.2%) to 95.28% when pulling bc16ce7 on sloria:create-handler into beba03d on crs4:develop.