Support configuration for GitHub baseUrl

[sradi]

Hi,

I tried to use Probot as a foundation for a GitHub App, which is going to be installed in our self hosted GitHub Enterprise instance. As GitHub App and GitHub Enterprise are on the same local network, I found it weird, that Probot forces me to use Smee (and it was complicated to setup behind a corporate firewall, etc.).

So I decided to use github-app and github-webhook-handler, as described here. But I couldn't make github-app work with a custom GitHub url.

Currently I am using a modified copy of the github-app, which adds the 'baseUrl' parameter to the constructor of @octokit/rest.

Maybe you could officially make the GitHub url configurable and pass it through to the @octokit/rest?!

Best regards Stefan

[sradi]

Maybe, github-app could read GHE_HOST from the environment and use it, if it is set?!