notmuch-show-get-header: new function; return alist of parsed header fields.

This function parses the displayed message to recover header
fields. It uses mailheader.el to do the actual header parsing, after
preprocessing to remove indentation.  It relies on the variables
notmuch-show-message-begin-regexp, notmuch-show-header-begin-regexp,
and notmuch-show-message-end-regexp.