Feature request: add Complement and Difference

[sammthomson]

Thanks for this library!
Complement and Difference are supported operations in OpenFst that would be incredibly useful to re-export here

[Garvys]

Hey !
They can totally be added. I don't have the bandwith for now so feel free to add it if you feel like it.
Else I will try to do it in the future

[bluebear94]

I would also like to have intersection.