REPL.TerminalMenus: Allow keybindings in RadioMenu

[IanButterworth]

Adds optional keybindings to REPL.TerminalMenus.RadioMenu
I decided not to automatically prepend the keybinding, to allow the user flexibility in formatting.

options = ["o: orange", "a: apple", "b: banana"]
keybindings = ['o', 'a', 'b']
menu = RadioMenu(options, pagesize=3, keybindings = keybindings)
choice = request("Choose your favorite fruit:", menu)

Choose your favorite fruit:
 > o: orange
   a: apple
   b: banana
2 # after pressing 'a'