Allow function for outFolder in dest

We use a PHP framework that stores assets within bundles. We want to use gulp to compile these assets but there are a large quantity of the bundles. It is obviously possible to set up a pipelines for each using a loop, but we would end up with 100's.

I propose allowing passing a function as `outFolder` to `dest`, so that the input file info can be used to manipulate the output path. I attempted to create a plugin to do this but it just ended up as a copy of `dest`, so decided a PR would be much cleaner. I think this feature will be useful to others, and doesn't add too much code.

It's only a proposal at this point so I haven't taken the time to write a test or document it, but if you are interested let me know and I will produce both of these.