Fix bug with string pattern matching with "."

Previously, a route like "/:a.png" would match "/foo/bar/baz.png".
This was incorrect, as all matches should be restricted to path segments
(or smaller, as dictated by a break character).

This bug was introduced in 1a390aba1c7c591e2c0c75be43eb830091b7d6f7.

Fixes #75.