Add support for runtime parameter values when using Message Handlers

[salsferrazza]

Currently, MessageHandler specific configurations must be hard-coded within the handler module itself. There should be a syntax for specifying user parameters at runtime through the command line syntax. Something like:

message_handlers=TimestampPullForwardHandler:timestamp_field=transact_time

Or potentially a <MessageHandlerSubclassName>.json can be automatically checked and the dictionary passed into the MessageHandler constructor? This might give the most flexibility without rendering the CLI syntax too unweildy.

[salsferrazza]

#129