Make rules extendable #50

[jaredlockhart]

Fixes #50

[coveralls]

Coverage remained the same at 100.0% when pulling 3430327 on JaredKerim-Mozilla:50 into 7e8b27f on mozilla:master.

[pdehaan]

I think this is fine, but I'm still trying to wrap my head around the shift from typeof metadataRule === 'function' to Array.isArray() and if the code is doing the same thing here.

[jaredlockhart]

It's a weird difference. Where previously we were looking for typeof function because we had already applied buildRuleset and so each 'rule' was a callable. But now we've pushed that off, and so we're looking for arrays which are lists of rules, and then we wrap them in buildRuleset and apply them, or recurse deeper.

[coveralls]

Coverage remained the same at 100.0% when pulling 9669500 on JaredKerim-Mozilla:50 into 7e8b27f on mozilla:master.

[pdehaan]

R+ Looks good to me (with the one question/concern noted above).
Logic looks the same as @k88hudson's earlier PR.

[coveralls]

Coverage remained the same at 100.0% when pulling b018ece on JaredKerim-Mozilla:50 into 7e8b27f on mozilla:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 0cd91e5 on JaredKerim-Mozilla:50 into 7e8b27f on mozilla:master.