Add helper for adding index based mappings from files

I just created [node-jsmin-sourcemap](https://github.com/twolfson/node-jsmin-sourcemap) and during its creation I needed index support from source-map.

As a result, I created the attached code to assist in my dilemma. I have included tests, updated the README, and bumped the patch version.

It should be a huge help for others when adding source-map's to their compressors/minifiers.