map form files

I needed this to map files in a form to strings in a structure so I added the function `mapFormFiles` that does that. It mimics hapi's payload parse.

Disclaimer: I don't know what I'm doing, never used reflect before, don't even know how it works. What I'd like this pull request to do is map a file in the form to some kind of io.Reader in a struct, but in my special case I would also like to accept the same kind of field as JSON string. It also has no idea if the file is a text file or not.

It's probably not ready to be accepted as is, but it's under way.