Features for easier explaining of reactivity

- makeReactiveBinding: Turns a "regular" variable into a reactive.
  No need to use reactiveValues() for simple reactivity.
- setAutoflush (not exported): Causes flushReact() to be called
  each time something is executed at the R console top-level.
- options(shiny.suppressMissingContextError=TRUE): Prevents the
  "Operation not allowed without an active reactive context" error
  when attempting to read a reactive value or expression from the
  console.