Find trees

Support the XPath `doc(...)` function and use it to locate trees outside of the application tree. This provides access to the `clients.vwf` client container or any other global tree.

@eric79, @allisoncorey, please review. @scottnc27603, @jessmartin, fyi.

I think it might be better to move those `or` into the `matches` function, inside the `some` anonymous function ... at which point, maybe `matches` goes away, and the logic just becomes part of this function.