Compile file

I've been wanting to add this API for ages, I just haven't got round to it.  It's a much cleaner expression of intent than either doing `jade.compile(fs.readFileSync(filename, 'utf8'), {filename: filename})`, which I write far too often.

As an added bonus, it would be fairly easy to write a transform (or update [jadeify](https://github.com/domenic/jadeify)) so that files that compiled jade in this fashion also worked client side.