docs: function(key, callback, options) {
          var binding;
  
          key = key.charCodeAt ? key.toUpperCase().charCodeAt() : key;
          options = assign({}, DEFAULT_OPTIONS, options || {});
          binding = { callback: callback, options: options };
  
          if (!this.keyBindings[key]) {
              this.keyBindings[key] = [binding];
          } else {
              this.keyBindings[key].push(binding);
          }
      }