Allow setting midje config via java system properties

[philomates]

addresses #482

allows for doing things like:

(System/setProperty "midje.check-after-creation" "false")

;; run Eastwood analysis...

(System/setProperty "midje.check-after-creation" "true")

this is important because things like Eastwood analysis will load a bunch of namespaces but we want to avoid executing midje facts tests because that slows things down / has side-effects.

Given a config key like :my-key, the way to override it via system properties would be to set "midje.my-key"