Extend support for Cloud Events

[tardunge]

Hi,
I'm very much liking the transporter and wanted it to support cloud events specification.

I have some pubsub subscriptions which deal with google generated cloud events and the data is base 64 encoded. Unfortunately i cannot specify any pattern here to filter out the messages of my interest.

Can you extend the support for this?
It will be based on an assumption that if the event payload follows the cloud event spec, then the data is base64 encoded. If this can be resolved by providing base64 decoder func in the event pattern decorator, that would be awesome.

Thanks for the effort.
Happy to contribute.