Support optional step inputs

[brandonvin]

greenlight.step/lookup does not allow any inputs to be omitted, so the typical workaround is to do something like the following, which hurts test understandability (step depends on more than just its :inputs) and increases test boilerplate.

#:greenlight.step
{:inputs {:some-required-data (step/lookup :some-required-data)}
 :test (fn [{:keys [some-required-data] :as ctx}]
         (let [some-optional-data (:some-optional-data ctx)]
		   ,,,))}