Allow graph to be directed

Hi,

I'm interested in using this work as a basic building block in my research. However, I require the graphs to be **directed**, and **edge contraction** to be only limited to **certain edges**. So if I understand correctly, I will need to first add the directed option in function find_embedding, and also add restrictions to edge contractions.

Any suggestion on where might be the best place for me to make these two changes? Thanks!