Fixes #419 - handling multiple input source maps.

Fixes #419 - handling multiple input source maps.

Adds support for multiple input source maps when data is passed in
as a hash, e.g.:

```
new CleanCSS({ sourceMap: true }).minify({
  'path/to/source/1': {
    styles: '...styles...',
    sourceMap: '...source-map...'
  },
  'path/to/source/2': {
    styles: '...styles...',
    sourceMap: '...source-map...'
  }
})
```

Effectively it is the same as concatenating source maps, but should
be less error-prone.

See 'multiple source maps' in `test/source-map-test.js` for examples.