Mention `--header-style` in readme and history
Keep `set_options` options alphabetized
Remove misleading header defaults
Misc style tweaks
Remove unneeded condition inside merge
Fix warnings of multiple display const definitions

Warnings appeared while running `twig gh-update`.
Tweak `--header-style` docs
Misc contributing updates

- Suggest opening an issue to discuss before
  starting a large branch.
- Suggest keeping a branch focused on a single
  topic.