Allow set multiple websocket handlers (in different paths)

Hi!

Currently, jet only supports one unique websocket handler, and would be awesome having a more flexible way to set multiple websocket handlers.

With approach implemented in this pr, allows set different websocket handlers in different paths.
Obviously, this pr is "unfinished", because I want to ask you if you would  include this change. In affirmative case, I will update pr with docs.

Additionally, supporting two ways to setup websocket handler may be confusing, and it has different approaches to  manage this situation:

- Explain that `:websocket-handler handler` is just alias for `:websockets {"/" handler}` and the last one has the priority..
- Deprecate `:websocket-handler` option and document it on the readme.

Thank you for the really fantastic package.