added very tentative sandboxed-preview example.
Added a 'preview plugin' architecture.

This makes it easier to support functionality that enhances the preview
area but also needs to talk to the editor, and which may or may not be
running in an environment where the preview frame is in a cross-origin
document.

The only preview plugin right now is the 'source-higlighter', aka
x-ray goggles effect.
remove unused dependency
Pass plugins into LivePreview constructor instead of via loadPlugin().

This allows remote LivePreview instances to load all their plugins
before signaling their ready state, so that weird race conditions don't
arise between the editor and preview sides of a plugin.
refactored sandboxed-preview example a bit.