Abstract out "analyzeTree" into separate probot extension

[swinton]

Via @bkeepers:

https://github.com/swinton/linter-alex/blob/master/lib/analysis.js

Some pointers:

• Existing extensions: https://probot.github.io/docs/extensions/
• Metadata extension: https://github.com/probot/metadata

Metadata usage:

const metadata = require('probot-metadata')

const kv = await metadata(context)
kv.set('edits', kv.get('edits') || 1)

Applied to analyzeTree

1. Needs context, filter, action
2. Use pagination helper to support more than 100 pages