Fetch or auto-discover contributors (auto-generate)

- `all-contributors-cli` version: 5.4.1

Problem description:

Currently, `add` let's you manually add contributors. However, for existing projects, it may be tedious and error prone to follow this manual approach; thereby creating a high barrier of entry for anyone who wants to start implementing the spec.

Suggested solution:

It'd be great to have a way of auto-discovering this information. E.g.

- commmitters -> code
- issue creators -> bug
- pull requests -> code

Github's [contributors API](https://developer.github.com/v3/repos/statistics/#get-contributors-list-with-additions-deletions-and-commit-counts) can help discover the first item; while [issues API](https://developer.github.com/v3/issues/#list-issues-for-a-repository) can take care of the remaining two.

If there is interest in adding this, I can help send a PR.