Creates and returns an agent with an initial value of state and zero or more options (in any order): :meta metadata-map :validator validate-fn :error-handler handler-fn :error-mode mode-keyword If metadata-map is supplied, it will become the metadata on the agent. validate-fn must be nil or a side-effect-free fn of one argument, which will be passed the intended new state on any state change. If the new state is unacceptable, the validate-fn should return false or throw an exception. handler-fn is called if an action throws an exception or if validate-fn rejects a new state -- see set-error-handler! for details. The mode-keyword may be either :continue (the default if an error-handler is given) or :fail (the default if no error-handler is given) -- see set-error-mode! for details.