New Feature: Full host and protocol support

This is an alternative implementation of #131 which is more full featured and includes breaking changes.

### New Features:
* Adds support for specifying a default protocol in `default_url_options`.
* Adds support for specifying a default host in `default_url_options`.
* Adds support for specifying a default port in `default_url_options`.
* Adds support for routes which specify a specific protocol.
* Adds support for routes which specify a specific host.
* Adds support for routes which specify a specific port.
* Adds support for using the `window.location.protocol` when a protocol has not been specified.
* Adds support for using the `window.location.host` when a host has not been specified.
* Adds support for using the `window.location.port` when a port has not been specified.

### Changes:
* `generate_url_link` no longer validates that the `:url_links` config value contains a protocol.
* Changes expected value of `url_links` to boolean true/false and deprecates the previous usage. Default protocol, host, and port can be configured with `default_url_options`.

This PR fixes #127.
@le0pard What do you mean by "moved in `context`"? It is already in a context. Do you mean that the tests should be refactored so that only the tests that generate URLs based on `window` should set the `window` object?

What should be the behavior when `window` is not defined? Can you provide an example use-case which requires this support? I don't know of a situation where we would need to support that. As far as I understand, this gem is only intended to work with a Rails app.