Create `new` class method on Symbol

[saulshanabrook]

It would be nice to have a new classmethod on Symbol like the one on Operation to make a subclass more succinctly. I think it would just take one argument, the classname.

[hbarthels]

I think that's a reasonable feature. Unfortunately, I don't have much time to invest on MatchPy right now. If you want to implemented that, you are more than welcome to do so.