cargo-tree: Mark proc-macros.

It would be good to separate proc-macros in the cargo-tree output. Either as an indicator to the side (like `(proc-macro)`, or a separate section like `[proc-macros]`. cc https://github.com/sfackler/cargo-tree/issues/64

