Add destpath to process callback (Ref #1161)

Change for issue #1161. There doesn't seem to be any reason not to pass the destination path to the process callback, since it is already available and it could definitely be useful in some cases. This change adds that. It isn't a breaking change, any gruntfiles and process callbacks not referencing the third argument will ignore it and work as usual.

Also see docs issue https://github.com/gruntjs/grunt-docs/issues/79 and PR https://github.com/gruntjs/grunt-docs/pull/80.