Support for gists URLs

[kolarski]

First, congrats for the great library.

As I was running trough a list of Github Urls, I stumbled upon a npm package: quarterto/cheerio-walk with the following Github URL:

git+https://gist.github.com/c4f67949eb47ea532dc27a6d8d93d274.git

I'm not sure if the URL is correct in the first place and if should be parsed like github url.
Currently the lib returns null for such urls.

[jonschlinkert]

I think it makes sense to support gist URLs. A PR would be great.