gulp-source-map Support

Adds support for gulp-source-map to create an external source map.

```js
.pipe(sourcemaps.init())
  .pipe(less({
    paths: [ 'assets/less/' ],
    sourceMap: true,
  }))
.pipe(sourcemaps.write('./'))
```
missing some code? the usage here looks different from https://github.com/wearefractal/gulp-coffee#source-maps