Use github api to build an async completion source for repository names

I was planning to add something to https://github.com/sigma/gh.el to support this feature. I'm almost done, and was wondering what you would think about doing something like this.

We might have to add helm as a (possibly optional) dependency to do something like this, since normal emacs completions don't really support async candidate generation.

I've made the changes to gh.el in sigma/gh.el#70, but I decided to write https://github.com/IvanMalison/github-search to implement this feature.

It would be really easy to have github-clone USE github-search's search feature
You could hook right into github-search-select-repository-from-search-string at

github-clone.el/github-clone.el

Line 128 in f513403

(list (read-from-minibuffer "Url or User/Repo: ")

or offer a separate interactive function that does so.

Alternatively, you could set github-clone as github-search-clone-repository-function

And use github-search to start the call