Support regexp routes with prefix.

In Dancer 1 it was possible to declare a route with a regepx in a
prefix, but that feature was not enable in Dancer 2.

Add the feature back and change a test to be sure it works.

Closes #254.
Update the test plan for t/route.t
reflect Changes
Merge branch 'franckcuny-pr/prefix-w-regexp-routes' into devel