Index maps support

**WARNING:** TOTALLY INCOMPLETE SO FAR!

This will hopefully fix #16, some day. Issuing PR to get conversation started & make sure I'm not going totally off the rails.

Started building out `Simple` & `Index` `SourceMapConsumers` that inherit from a base `SourceMapConsumer` that defines shared behavior. Not much progress yet althrough I do have a `SimpleSourceMapConsumer` passing the basic "this thing can be instantiated test".