Add support for dynamic segments in visitable urls

[drewchandler]

This allows you to do the following:

let userPage = PO.build({
  visit: PO.visitable('/users/:id')
});

userPage.visit({id: 5});

[san650]

This is great! thanks for contribute.

I plan to update the addon to ember-cli 0.2.6 today and make a new release with this feature.