Add support for sourcesContent property in source maps

The [source map specification](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit?hl=en_US&pli=1&pli=1) describes an optional field `sourcesContent` in the source map. This field comes in handy if you don't want to load source files separately when you're using the source map: the actual content of these in included in the source map directly.

This PR implements this feature using new option `sourceMapIncludeSources`.

Fixes #132