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`?


You can't pass in aArgs to the sub source map's generatedPositionFor directly; first, you need to subtract its section's offset.

The idea is that an indexed source map is just a concatenation of existing source maps, and you shouldn't have to offset every source map in the indexed map when generating it.