New Feature: Protocol support

### New Features:
* Adds protocol support when generating URL helpers, for routes with specific protocol specified.
* Adds support for "protocol relative" base URL value in `:url_links` config.

### Changes:
* `generate_url_link` no longer validates that the `:url_links` config value contains the protocol.

This PR fixes #127.
Yes, @bogdan is right. `//example.com` use protocol from which web page was loaded.