Allowing for custom :value for original-content. Closes #364

Pretty simple tweak. However I've also encountered this: what's the distinction between `bip-original-content` and `bip-value`? They both pull the same stuff. Should `bip-value` also have this `opts[:value]` applied to it? I'd think probably?
(defaults is the field's value)

object.send(field) can be confusing for new rubists.