Add #read_keypress to Shell

Sometimes we want to read a single keypress from the user. This will
read any keypress, no matter how many characters it is.

  require "tty"

  shell = TTY::Shell.new
  key = shell.ask("Press a key: ").read_keypress

  case key
  when "\e[A" then puts "Up"
  when "\e[B" then puts "Down"
  when "\e[C" then puts "Right"
  when "\e[D" then puts "Left"
  ...
  end

Fixes #10.