Support multiple targets in useminPrepare

Inspired by #162. I tried that pull request locally but it didn't work. I guess it was broken when @trask squashed the commits (I can see some discussion with @sindresorhus on lines which were not in the squashed commit there). :(

I also looked at the first pull request from @trask, #143, and there were too much unnecessary changes IMHO. This pull request just gets the files list from ```this.filesSrc```.