Allow a regex filter for RUST_LOG

When specifying RUST_LOG, the programmer may append `/regex` to the end of the spec. All results will then be filtered using that regex.

r?
Maybe this could just be a normal list, since, e.g. these Rust comments are out of place:


> - `hello/foo` turns on all logging for the hello module, where the message includes `foo`.
> - `info/f.o` ...