Add support in SourceMapConsumer for indexed source maps

Fixes #16 (or at least the consumption part of it)

This is a WIP, but consumption of indexed source maps seems to be working. Let me know if you see anything going complete the wrong way, or if there are additional things that need implementing that I'm missing.

I'm not totally sure where the documentation for each of the functions should live - should I create stub functions on `SourceMapConsumer.prototype` that just throw errors saying they must be overridden, and place the documentation there? Or is it okay to duplicate the documentation between `BasicSourceMapConsumer` and `IndexedSourceMapConsumer`?


Is there a reason `_parseMappings`, `_generatedMappings`, and `_originalMappings` don't only exist on `BasicSourceMapConsumer`s?

First of all, indexed maps don't have mappings themselves, so what parsing of mappings do they need to do?

Second of all, this would enable us to avoid parsing every mapping when we need a location in just one of the sections. Parsing the mappings is pretty expensive and frequently dominates time spent in profiles; I'd like to keep it as lazy as possible.