GeSHi support

Why is this patch needed:

1. Because GeSHi does some preparation and wraps its code blocks in a square bracket notation, markdown sees it as a simple paragraph. Thus stuff inside gets escaped etc.
2. To call the GeSHi filter before marksmarty is not a solution, because code blocks generated by markdown won't get GeSHi features then.

So when activating the GeSHi support, one should call Markdown before GeSHi to get this:

    a generic markdown code block
    with linenumbers thanks to GeSHi
    this is plaintext mode

<blockcode type="php">
$syntax_highlighting = true;
</blockcode>

<blockcode>
this codeblock is in plaintext mode,
just like the generic markdown code block!
</blockcode>