Add optional tracing feature, trace which logic

[Xaeroxe]

This also moves regex into a proper feature.

Fixes #95

[ActuallyHappening]

This is really good!
I was also hoping for some debug!() level emits for higher level functionality, like if I called which("cargo") it would emit (as well as trace) some debug!("Which found path "/Users/ah/.cargo/bin/cargo" from query "cargo") or something

But this can be added in a point release now anyway!