Add sourceMap option.

- sourceMap enabled, will write a source map of the concatenated file.
- sourceMapName option controls the name of the exported source map.
  - sourceMapName can be a string or a function
- sourceMapStyle option can be `link`, `embed`, or `inline`
  - as `link`, source map will refer to concatenated files
  - as `embed`, source map will embed file contents
  - as `inline`, source map will be a data url in the concatenated file

If stripBanners or process is enabled, sourceMapStyle == embed or
inline is required. This way the source presented using the map will
be accurate.