Local webhook development: A Guide to customizing smee.io for fun and profit

Original title ("How We Converted a GitHub Tool Into a General Purpose Webhook Proxy to Supercharge Our Integration Development") was too long, and just trimming it was losing context, I hope it's within the rules that I got a little creative with the shortening.

Background: We used smee.io for local github app development, and wanted to use it also for slack app and other webhook based local development, we added a few enhancements and though it might be relevant for others as an alternative (for some use cases) to using ngrok.