Add root configuration option for relative routing

[svicalifornia]

Most APIs have a common prefix in all of their routes, e.g. /api/v1/

Repeating this prefix in all Pretender routes is less than ideal. It's not DRY, and an API prefix can sometimes change, requiring redundant edits across all routes.

It would be great if we could configure a shared root and specify the routes relative to this root, as shown below:

const server = new Pretender();
server.root = '/api/v1/';
server.get('users', ... );
server.get('users/:id', ... );
server.get('products', ... );
server.get('products/:id', ... );

[svicalifornia]

Even better, this root property could be used to limit which requests will be handled by Pretender, so that requests to other URLs that don't match would automatically pass through.