Allow pipeline handlers to be instantiated with a fixed context

[rtibbles]

Currently handlers in the file pipeline take context as an argument to the handle_file method in the form of keyword arguments that are specifically filtered using the CONTEXT_CLASS for the handler.

It would be useful to be able to instantiate the handlers with those arguments, allowing the context to be defined at initialization time for the pipeline, rather than being passed in on a per invocation basis.

This would allow the pipeline to be more directly configurable, and not require a global context object that combines all of the configurations for each of the constituent handlers.

This should be possible to do in a way that still allows for per invocation override of the context.