Show the current Pry prompt before console is used

[RobinDaugherty]

Currently, the console prompt is always ">>". Would be great to show the current Pry prompt instead, which depends on the currently-selected frame. For example, [1] pry(#<HomeController>):1>.

This will require a change to the Better Errors error page.
It seems like the easiest way to get this will be to have Better Errors call do_eval when the frame is changed.

An alternative would be to have the REPL instantiated for every frame as the page is being built.
This doesn't seem like it would be efficient.