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.
Yes, the regexp is correct (see the spec).

It is actually inserted into the generated regexp, so `\A` and `\z` would mess everything up.