ngrok 1.6 (new formula)

This pr adds a brew for ngrok[1], a tool that allows you to securely expose
a local web server to the internet and capture all traffic for detailed
inspection and replay. The formula is Go-based and depends on Go,
Bazaar, and Mercurial.

This formula is a new from-source PR as requested by @mikemcquaid on
issue #20839 - essentially a resubmission of #23732 which has an
unresponsive author.

[1]: https://ngrok.com