Adds optional `sourcesContent` Support

This commit adds the ability to inline all sources that are concatenated into the `sourcesContent` property of the generated sourcemap. The advantage to this option is that it removes the need for developers to upload the original src files to the webserver that will be serving the source map file.

This uses the `setSourceContent` API defined in https://github.com/mozilla/source-map#sourcenodeprototypesetsourcecontentsourcefile-sourcecontent