Limit binding generation to certain headers

[adetaylor]

After #974, we should be apply to apply a filename filter to the items generated e.g. just generate items in "foo/bar".

We should also be able to specify a Rust package root to learn about types relating to other packages.

This will enable multiple include_cpp! macros to work together.

[adetaylor]

(maybe the second part of this is related to #967)