Liquid Tags: Multiline regexp & graphviz support

Hi, 

in order to add some graphviz liquid tag, I had to first add *multiline* regexp support to liquid tags. Now I'm able to add liquid tags which match more than one line. 

In addition to that I've added a simple *graphviz* liquid tag. Some examples:

```
{% graphviz 
    dot {
        digraph graphname {
            a -> b -> c;
            b -> d;
        }
    }
%}


{% graphviz 
    twopi {
        <code goes here>
    }
%}
```

Please let me know if you have futher questions. 

Cheers, Victor