Support configuring multiple organizations centrally

## Prerequisites:

* Is the functionality available in the GitHub UI? If so, please provide a link to information about the feature.
Yes, no pertinent link

* Is the functionality available through the GitHub API? If the functionality is available, please provide links to the
API documentation (https://developer.github.com/v3/) as well as the Octokit documentation (https://octokit.github.io/).
Yes, no pertinent link

* If the functionality is not yet available in the API, it would be helpful if you
contacted support (https://support.github.com/) or posted in the Community Forum (https://github.community/). Please
include a link to the forum post if you create one or a copy of the response from support.

## New Feature

Working with enterprise contexts, I think it would be interesting to be able to manage multiple orgs from a single Safe-Settings instance and a single admin repository. It would allow central management of repository in enterprise context, where you may not want to split these configs between orgs.

Although this may seem major, I think it actually may be quite minimal as a code change. I think updating `index.js` could be sufficient to add this use case. For example we could have the following paths: `.github/ORG_NAME/repos/...`. Basically keeping the same code but splitting at the base using the ORG_NAME to select the installation id it needs to apply the changes to.

We would need to make sure it doesnt change the flow of the existing org-centric deployments, so maybe with a env var switch or by using a different file structure that does not intersect with the existing one.

I might have time to develop something like that, but I do want your comments / opinions on this before I start writing a PR, this is still a pretty major feature.

@decyjphr for visibility