Implement Occur multi-line matches.

Implement Occur multi-line matches.
http://lists.gnu.org/archive/html/emacs-devel/2010-03/msg01044.html

* replace.el (occur): Doc fix.
(occur-engine): Set `begpt' to the beginning of the first line.
Set `endpt' to the end of the last match line.  At first, count
line numbers between `origpt' and `begpt'.  Split out code from
`out-line' variable to new let-bindings `match-prefix' and
`match-str'.  In `out-line' add non-numeric prefix to all
non-first lines of multi-line matches.  Finally, count lines
between `begpt' and `endpt' and add to `lines'.