Support for SSL configuration in cable.yml

[w pp]

Hello,

AFAIK when you intend to run a redis connection via SSL you have to overwrite

`ActionCable::SubscriptionAdapter::Redis.redis_connector`. My current solution

looks something like:

```ruby

cert = File.read config[:ssl_params][:cert]

key  = File.read config[:ssl_params][:key]

config = config.merge(

  ssl_params: {

    ca_file: config[:ssl_params][:ca_file],

    cert:    OpenSSL::X509::Certificate.new(cert),

    key:     OpenSSL::PKey::RSA.new(key)

  }

)

```

which lives in an initializer.

I'm wondering if a change to the redis subscription adapter, which would allow a cable.yml configurations like:

```ruby

production:

  adapter: redis

  url: rediss://redis-cluster

  ssl_params:

    ca_file: /certs/ca.pem

    cert:    /certs/cert.pem

    key:     /certs/key.pem

```

would be a welcome addition to the rails/action-cable? If that's the case I'd be happy to start on a patch. If this is more

of an edge case and the change is not welcome, I understand but would appreciate some feedback on my current approach

and would like to submit a patch to at least mention this option in the [documentation](http://edgeguides.rubyonrails.org/action_cable_overview.html).

Thanks for your time,

Have a great week.