Add support for immutant

[kaosko]

I was experimenting with Sente in a re-frame app. However, Sente's api felt a bit clunky and I found I still needed to write some boilerplate/middleware for "re-framing" the messages to events. Then looked for alternatives and found your tubes, which seems to have a nice and clean real-world api. I'm using Immutant though, would you consider adding an adapter for it? Seems like it'd be relatively simple to write another websocket-handler, similar to Sente's Immutant adapter (https://github.com/ptaoussanis/sente/blob/master/src/taoensso/sente/server_adapters/immutant.clj).

[drapanjanas]

Sure, I think it would be possible. But, unfortunately I'm quite busy at the moment and not familiar with Immutant, so would not spend time on it in closest future. Though, I would merge PRs if any and release.