This helps avoid a gotcha when using matches with sub-routers.

Fixes #120.

Go's regular expressions don't allow you to create a capturing group
named "*", which previously made using SubRouter with regular expression
patterns impossible. This change introduces the alternate key "_", which
happens to be a legal capturing group name.

Fixes #98.

This middleware makes it much easier to write sub-routes, allowing you
to make the sub-router ignorant of its parent router's matched prefix.

The example app has also been modified to use this functionality for its
admin pages.

Fixes #65.