Cleaner constructor

[lucasoshiro]

Hi!
First of all, thanks @mahdavipanah for this lib, this helped my a lot when I was studying context-free grammars.

However, I that this could be a easier to use, with a cleaner and more flexible constructor, like this:

from cfg import CFG

g = CFG(terminals={'a', 'b', 'c', 'λ'},
        rules={'S': ['aSa', 'bSb', 'cSc', 'λ']}
        )

I've implemented this in this code for my personal use, I thought it would be fair to share it with you. This is still compatible with the current constructor.

Tests are included to make sure that everything is ok.

[mahdavipanah]

@lucasoshiro I'm glad that this project has been useful for you. I'm sorry for not getting back to you sooner, but this project is from a couple of years ago and I don't have the time to maintain it anymore. I appreciate your pull request. If you can take a look at these issues and potentially create a pull request to fix them, it would be awesome:

• .cyk() method of CFG class appears to be incorrect sometimes. #1
• Error on a CFG #2