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.
I suppose the logic branching could start with assuming if a custom protocol was passed as an argument, use that instead. But I think that decision needs to be made by the owner/maintainer of this gem since it would change the current behavior/expectation of the config value.