Pull in Mustermann

[Mustermann](https://github.com/rkh/mustermann) is the new pattern matcher for Sinatra 2.0. It can also be used as a plugin for Sinatra 1.x, assuming you are running on Ruby 2.0 or later.

Using Mustermann will fix #131.
Ah, okay, I wasn't quite able to tell from the Mustermann docs if Mustermann would anchor the regex correctly, or just check if the `:id` param matched the regex.

Still wondering if there's an issue if someone creates the repository `12345/settings` or something like that on GitHub.