Support `forbid` lint level

[Kromey]

Like deny, forbid turns lints into errors; unlike deny, however, forbid cannot be overridden. https://doc.rust-lang.org/rustc/lints/levels.html

For projects or organizations that want to take a "hardline" stance on certain lints (e.g. unsafe_code), having cargo-cranky support the forbid lint level could be very important.

(NB: There's also a force-warn lint level that is a non-overridable counterpart to warn, and may also be desirable in some instances.)