Make bon builders optional

[nicoburns]

Objective

Reduce compile time / binary size impact of Comrak for projects that are not otherwise using the regex crate.

Changes made

• Add bon feature flag
• Make bon builder derive/attributes conditional on this feature flag

Notes

• Bon is particularly heavy on dependencies (bringing in syn, darling and more), and is not that widely used so is likely to be something that projects could eliminate from their dependency trees entirely.
• With this PR + Eliminate regex and once_cell dependencies. #514 + Eliminate regex dependency unicode-rs/rust-caseless#20:
  • A release build of Comrak with --no-default-features build in 1.9s
  • The headers example (with default features disabled) produces a ~700kb binary

[kivikakk]

Thank you, again! This is a huge reduction in build-times. 🎉