Snapshot(s) on demand

[oliyh]

Would be nice to be able to write something like :kamera/snapshot to the js console and have kamera take the snapshot and do the assertion.

Would need to know in advance that this was a devcard with that behaviour, and also how many snapshots to expect, e.g.

{:capture-on :console ;; new option - other values could be timeout, or dom selector
 :capture-count 3 ;; expect 3 captures, number them in order when looking for comparison file
}