Add required app permissions to routes

[gr2m]

We can scrape this page:
https://developer.github.com/v3/apps/permissions/ and it's GitHub Enterprise (Server) counterparts

[MaximDevoir]

I think this could also be utilized by Probot. The larger goal with helper features like webhook-event-check is to help developers catch permissions-related issues with their GitHub App faster.

With permissions information for each route, Probot could be able to compare the permissions received from GET /app to the ones required with each request. I've got a few other alternative solutions I'll explore when it gets to that.

I'll explore this codebase and try to understand how everything works and then work on a PR for this issue. My goal will be to get a PR posted by the end of January February.

[gr2m]

Holler if you have any questions