Add basic source map support

This PR addresses #1, but with some caveats:

The scenario in which this works correctly is one where the source file, compiled file, and source map are all in the same location. Additionally, since this plugin will stream two files for every one input, subsequent commands may need to pass through when the source map is streamed.

The source map and the compiled source both contain name references to each other (which act as relative paths), so scenarios where the source map and compiled source are not written to the original source file's location will not work.