Add sourceMap option.

- sourceMap enabled, will write a source map of the concatenated file.
- sourceMap can be enabled with an object with the following options:
  - content: If true, store the content of the source files in the map.
  - root: A string stored in the source map as sourceRoot.
  - inline: If true, instead of writing to disk, source map is a
    datauri embedded in the concatenated file.

If stripBanners or process is enabled, sourceMap.content is required.
This way the source presented using the map will be accurate.