Bendoh streams

This is related to [gulp-ruby-sass issue #123](https://github.com/sindresorhus/gulp-ruby-sass/issues/123), and introduces a 'streams' option which will allow users to specify alternate pipelines for output files matching provided globs.

Example,
```
	intermediate({
		output: relativecompiledir,
		container: options.container || 'gulp-ruby-sass',
		streams: {
			'*.map': function(map) { map.pipe(gulp.dest('maps')); }
		}
	}, function (tempdir, cb, vinylfiles) {
```

The benefit of this is that it allows gulp modules that require intermediate, but might emit two different output file types, can split the stream into two pipes for each file type, which can be dealt with distinctly.

This is fully backward-compatible.