Inline graphviz filter

I've made some changes to allow a graphviz filter to be displayed inline (with other filters).
As part of this patch, I've also moved the loading of the PEAR module into the process function, just to improve PHP loading performance a little.

The difference now is that you need to enter the graphviz like this:

[graphviz]
/*
* @formats = gif
*
*/
digraph G { 
a -> b -> c  
d -> a  
d -> c    
}
[/graphviz]

But it does mean you can create output like the attached image.