Added source map support

So, not really happy with the state of source URL-based debugging (`eval`? blergh!), I spent a bit of time adding source map support.

This replaces the `includeSourceURL` option with `includeSourceMap`, which places a proper source map in the same directory as the file.

The code is ~~a bit~~ ugly, but it's smart enough to ignore the newlines added by a template when mapping original lines to output lines. It relies on Mozilla's source-map library, as well as a concat strategy based on [grunt-concat-sourcemap](https://github.com/kozy4324/grunt-concat-sourcemap), to handle both concatenation and source map generation.