Improve API for WAMP message options

[danmarcab]

Right now, if we want to pass options to the wamp message we have to use an options key:

{:ok, subscription} = Spell.call_subscribe(subscriber, "com.spell.my_topic_prefix", options: %{match: :prefix})

Quoting @jtmoulia
The justification for having an specific options keyword arg is that it maps directly to the WAMP message's options argument.

A suggestion to improve the API, would be:

Spell.call_subscribe(subscriber, "com.spell.my_topic_prefix", match: :prefix)

See #35