Changed concat to accept configurations in gruntfile

We had a concat problem trying to concat both js and css files. We needed to set the concat.options.separator for js files only. Which we couldn't because we were also messing with the css files. What we needed was something like this (in Gruntfile.coffee):
```
concat:
	'build/app.js':
		options:
			separator: ';'
```
Which means we only want the separator in the targeted file, not all usemin generated target.

What was changed was a way, with backwards compatibility, to allow the addition of other options in the concat process generated by usemin.