Add optional license detection

[Jake-Shadle]

cargo-deny has been using https://github.com/jpeddicord/askalono for years, but that crate is basically abandoned/unmaintained (no changes in 2 years) and has had minor annoyances that have needed to be worked around previously, but now is showing other issues that mean even more workarounds are needed. I've previously thought about basically copying askalono into this crate as an optional feature so that I can just get rid of the askalono dependency so that improvements/fixes are easier to roll out.