expose "dest" attribute so plugins can use it

The `dest` variable that is set for each individually rendered path is useful for some plugins.

For example, I'm writing a pjax plugin which hooks into `after_write` and outputs non-layouted files alongside the ones that Stasis is creating.

I either have to determine the destination file path myself (which requires plugging in to Tilt and seeing if the file has a supported extension), or use `@stasis.dest` from inside my plugin.

I admit that `dest` might not be the best name for this if we're exposing it to external plugins, but `destination` is already used and I didn't have any other ideas.

Feedback welcome.