Bug 870543:  Remove data-l10n-args
Follow-up to bug 805234 - Reworded the part about registering callbacks
Bug 868677 - DOM injection, part 1: Bound localize

Context::localize now returns a `bound` object which has an `extend` method.
This method can be used to add entities which will be observed for globals
changes that trigger the callback passed to `localize`.
Bug 868677 - DOM injection, part 2: HTML bindings

The HTML bindings now make use of the `bound` object returned by
`document.l10n.localize` to localize nodes that are attached to the DOM
dynamically.

When a node is inserted into the DOM, you can call `document.l10n.localizeNode`
on it to add all localizable elements inside the node to the document-wide
`localize` which is used to localize <body>.