transit format should allow for custom readers/writers

Currently it's not possible to bring custom readers and writers to the transit-formatter unless you define a whole new format.

(def my-transit (otarta.format/create-transit {:read-handlers additional-read-handlers :write-handlers additional-write-handlers}))

;; alternatives:
;; - (fmt/create :transit ...)
;; - (fmt/transit)
;;   - breaks existing (not really as `:transit` is probably used to identify the format)
;;   this would imply moving all over to defn's