Helper base class for Broccoli plugins that map input files into output files one-to-one.

Documentation to follow.

Can this help with compilers that are almost 1:1, like a minifier that takes a .js and .js.map file and outputs a .js and .js.map file?

Not at the moment. I don't know yet how to implement this and still have the API look beautiful. We also have to make sure that caching works correctly, as we have to invalidate if either the .js or the .js.map file changes. My plan is to write a source-map-aware uglifier plugin to understand this use case better, and then extract common code back into this Filter base class.