Feature: add arg to find-plans that limits number of plans returned (shop classic)

[mpatsift]

Add an arg to find-plans that limits the number of plans returned.

[mpatsift]

Two poss implementations:
(1) Add plan-num-limit arg (default value 1) which limits number of plans returned when :which is :first or :random.

(2) Same as (1), but plan-num-limit applies to :all and :random. (plan-num-limit defaults to nil.)

I have draft implementation of (1), but (2) may be more obvious to callers.

[rpgoldman]

1 sounds fine to me -- will you please make a MR for this, and we will get it merged for this project.