Allow config/cequel.yml to specify replication and durable_writes settings

The rake task cequel:keyspace:create does not provide a method to pass in options when creating the keyspace.

This adds support to replication and durable_writes settings on per environment basis.

@becomingwisest proposed #152 but it's missing durable_writes option and tests.
This is the mechanism for pulling the configuration from `cequel.yml` if it's provided, right? It might be clearer to do something like

```ruby
options = options.reverse_merge(keyspace.configuration)
```

At the top of the method, to avoid looking in both `options` and `keyspace.configuration` in multiple places (e.g. also on line 59)