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.

notmuch.el

@@ -225,6 +225,30 @@ Unlike builtin `previous-line' this version accepts no arguments."
     (re-search-forward notmuch-show-tags-regexp)
     (split-string (buffer-substring (match-beginning 1) (match-end 1)))))
 
+(defun notmuch-show-get-header ()
+  "Retrieve and parse the header from the current message. Returns an alist with of (header . value)
+where header is a symbol and value is a string.  The summary from notmuch-show is returned as the
+pseudoheader summary"
+  (require 'mailheader)
+  (save-excursion
+    (beginning-of-line)
+    (if (not (looking-at notmuch-show-message-begin-regexp))
+	(re-search-backward notmuch-show-message-begin-regexp))
+    (re-search-forward (concat notmuch-show-header-begin-regexp "\n[[:space:]]*\\(.*\\)\n"))
+    (let* ((summary (buffer-substring-no-properties (match-beginning 1) (match-end 1)))
+	  (beg (point)))
+      (re-search-forward notmuch-show-header-end-regexp)
+      (let ((text (buffer-substring beg (match-beginning 0))))
+	(with-temp-buffer
+	  (insert text)
+	  (goto-char (point-min))
+	  (while (looking-at "\\([[:space:]]*\\)[A-Za-z][-A-Za-z0-9]*:")
+	    (delete-region (match-beginning 1) (match-end 1))
+	    (forward-line)
+	    )
+	  (goto-char (point-min))
+	  (cons (cons 'summary summary) (mail-header-extract-no-properties)))))))
+
 (defun notmuch-show-add-tag (&rest toadd)
   "Add a tag to the current message."
   (interactive