dot2tex does not support label font formatting

[emilienkofman]

The dot languages supports bold/italic/superscript/subscript/underlines text for node labels. I forked the latest dot2tex and apparently it does not support font formatting.

Example graph:

digraph G {
    Foo[label=<<B>Hello</B><SUP>world</SUP>>]
    Bar[label=<<U><I><S>Hel</S>lo</I></U><SUB><B>world</B></SUB>>]

    Foo->Bar
}