feature: allow Loofah-ization of an existing Nokogiri document or fragment

[flavorjones]

Currently, the following code will add scrubbing behavior:

doc = Nokogiri::HTML HTML
doc.extend Loofah::ScrubBehavior::Node

but that doesn't include #text or the node and node set decorators.

So, I'd suggest that we allow Loofah initialize methods to receive a Nokogiri document / fragment and do the extension there. Then we should check that we're not relying on the typeiness of the Loofah document / fragment.

(And, actually, can we then eliminate the classes entirely, and just use extended Nokogiri docs everywhere?)

[flavorjones]

see branch mixin-behavior