Add support for router root specification.

[totherik]

Currently, if someone chooses to use enrouten with a particular root path, routes will not resolve correctly.

This PR adds support for specifying a base route when using this module.

app.use('/foo', enrouten({ /*... */ }));

and effectively is the same to doing

app.use('/foo', app.router);

[pvenkatakrishnan]

👍 Looks good.

[grawk]

It may be worthwhile to add a test case to enrouten to verify that when invoked with a root path app.use("/root", enrouten({})), any scanned routes ("/root/route1") aren't also available at "/route1".

The code changes look good otherwise.

[totherik]

@grawk PR updated to reflect test case.