Fix source code example bug with plain list export to HTML

Fix source code example bug with plain list export to HTML

* lisp/org-exp.el (org-export-format-source-code-or-example): Mark examples
by a property. o
* lisp/org-html.el (org-export-html-close-lists-maybe): Check if raw
HTML stuff was actually made from an example

Daniel Mahler writes:

> 2. I would like to embed source blocks in numbered lists, without
> breaking the numbering ie:
>
>    1) get ready
>     #+BEGIN_SRC sh
>    get_ready
>     #+END_SRC
>    2) go
>     #+BEGIN_SRC sh
>    go
>     #+END_SRC
>
>    currently the src blocks cause the numbering to reset, so all
> items in a sequence like this are numbered 1

This patch fixes this issue - but I cannot say anymore why the code in
org-export-html-close-lists-maybe does in fact work.  The code looks
wrong, but it seems to work.  What looks wrong is that i does not
check for the true indentation in the case when the line is not
protected.  It must be that this case is covered by some other code
further down in the exporter.