Support multiple targets in useminPrepare

It would be useful for me to be able to have multiple targets for the useminPrepare task.  It looks like multiple targets are supported for the usemin task via https://github.com/yeoman/grunt-usemin/pull/58, but not for useminPrepare yet.  Please review this PR, if you are willing to accept let me know and I will add some tests before you merge.  The code for maintaining backwards compatibility is a little hacky (see var simpleStyle), let me know if you have better idea for that code.  Thanks.