Allow different hostnames for login and validate

[jonocodes]

Some network setups have different DNS routes for the login URL and the validation URL. omniauth-cas assumes there is one FQDN for both of these routes. Looking at the code it always appends the host, instead of allowing this to be overridden:
https://github.com/dlindahl/omniauth-cas/blob/master/lib/omniauth/strategies/cas.rb#L155

However other CAS clients allow for this flexability, for example:
https://github.com/rubycas/rubycas-client#a-more-complicated-example

This way I could specify something like
:cas_base_url => "https://external.route.to.cas.com/",
:login_url => "https://external.route.to.cas.com/login",
:logout_url => "https://external.route.to.cas.com/logout",
:validate_url => "https://internal.cas/proxyValidate",

[dlindahl]

I no longer have access to a CAS server and therefore am no longer fit to maintain this project.

If you would like to volunteer to be a maintainer of this project, please let me know by opening an Issue.