notmuch.el: Refactor citation markup. Variables for minimum size, button text.

This is a fairly intrusive rewrite.

- I pulled the common code for the signature and citation case out
  into a separate function. This is not so much shorter, but I think it
  will be easier to maintain.

- I replaced the sequence of (looking-at blah) (forward-line)  with a single
  re-search-forward per citation.

New variables

- notmuch-show-signature-button-format, notmuch-show-citation-button-format
  Allow customization of button text.

- notmuch-show-citation-lines-min
  Do not buttonize citations below the given threshold.

Reviewed-by: Kan-Ru Chen <kanru@kanru.info>