Support sourcemaps from multiple files (Fixes #8).

Here's my shot at this. Let me know what you think. I think the core of the issue is that inline-source-map's `. addMappings` function takes a filename, but the mappings already have a filename associated with them. Associating them with the name of the file containing the sourcemap doesn't make sense. It has worked up to now because single-file transforms don't change the filename, but when working with multiple files, it is an issue. In the short-term I just worked around that by passing the mapping's name directly.
I'd rather fix this inside `inline-source-map`. Should be possible right?
So for now we can leave this workaround, but I'd love a PR to fix the bug in the right place.