move misplaced section heading
make git-commit-known-pseudo-headers customizable

Make the previously constant `git-commit-known-pseudo-headers'
customizable.  Replace the variable `git-commit-mode-font-lock-keywords'
with a function by the same name.

Tested-by: Jonas Bernoulli <jonas@bernoul.li>
add "Suggested-by" pseudo commit message header

Suggested-by: Myself I think <jonas@bernoul.li>