rewind - Undo and Redo for 'shiny' Applications

Adds a user-facing undo and redo history to 'shiny' applications. Application state, comprising registered inputs and optionally server-side reactive values, is captured as the user interacts with the application. Users may then step backwards and forwards through that history with the keyboard, with buttons, or by scrubbing a visual history rail. Rapid successive changes, such as dragging a slider, are coalesced into a single history entry, and related changes may be grouped explicitly into semantic steps.