Experimental, subject to change.

Needs `clj-slack`, Ref. <https://github.com/julienXX/clj-slack>.

Returns a signal handler that:
  - Takes a Telemere signal (map).
  - Writes the signal as a string to specified Slack channel.

Can output signals as human or machine-readable (edn, JSON) strings.

Default handler dispatch options (override when calling `add-handler!`):
  `:min-level`  - `:info`
  `:rate-limit` -
    [[5  (enc/msecs :mins  1)] ; Max 5  posts in 1  min
     [10 (enc/msecs :mins 15)] ; Max 10 posts in 15 mins
     [15 (enc/msecs :hours 1)] ; Max 15 posts in 1  hour
     [30 (enc/msecs :hours 6)] ; Max 30 posts in 6  hours
     ]

Options:
   `:output-fn` - (fn [signal]) => string, see `format-signal-fn` or `pr-signal-fn`
   `:conn-opts` - Map of connection opts given to `clj-slack.chat/post-message`
     Examples:
       {:token "MY-TOKEN"}
       {:token "MY-TOKEN", :api-url "https://slack.com/api"}

   `:post-opts` - Map of post opts given to `clj-slack.chat/post-message`
     Examples:
       {:channel-id "C12345678", :username "MY_BOT"}

Tips:
  - See `clj-slack` docs for more info on its options.