function end(str) {
      var res = handleTabCounts.call(self, str, freezeTabs);
      cb(undefined, res);
    }
Handles tabbed autocompletion.

- Initial tabbing lists all registered commands.
- Completes a command halfway typed.
- Recognizes options and lists all possible options.
- Recognizes option arguments and lists them.
- Supports cursor positions anywhere in the string.
- Supports piping.

@param {String} str
@return {String} cb
@api public