Add Lookbehinds to the Cheatsheet

[Lonniebiz]

Please add these two entries into the RegEx Cheatsheet? :

1. (?<=abc) positive lookbehind
2. (?<!abc) negative lookbehind

Here's where I'm talking about:

[Lonniebiz]

I attempted to add these myself here: https://github.com/gskinner/regexr/commit/8052afa8545d4bdd49326b9b5f8fc19dded1c8d2

Are there other steps I need to do?

[theblang]

@Lonniebiz Could you open up a PR with that change? I'd love to have it as well, I always forget.