v0.14.0

• BREAKING CHANGE: Remove many trait impls from util::Flag.
  This type had a number of deref and operator impls that made it usable as sort-of-a-boolean.
  Real-world usage showed this type is more useful if it's able to carry a span for good errors,
  and that most of those impls were unnecessary. #179
• Remove need for #[darling(default)] on Option<T> and Flag fields #161
• Improve validation of enum shapes #178
• Bump proc-macro2 dependency to 1.0.37 #180
• Bump quote dependency to 1.0.18 #180
• Bump syn dependency to 1.0.91 #180