builtin/search/github: transparently rate limiting requests

[mccurdyc]

What would you like to be added: Right now, the number of desired results is static at 10. However, if a user requested something like 1000 projects, it would most likely hit GitHub's API for 1000 projects at once and exceed per-minute rate limits (30/min)and neighbor would exit. Instead, neighbor should build this in to the GitHub search package.

Why is this needed: Neighbor will always exit before successfully evaluating all projects for a larger number of desired results.

Definition of done

When the GitHub search package can successfully obtain larger number of projects without exiting.

Links to documentation or design (if any) https://developer.github.com/v3/search/#rate-limit