'Toggle whether plays run with become'

def do_become(self, arg):

    if arg:
        self.options.become = boolean(arg, strict=False)
        display.v(('become    changed    to    %s' % self.options.become))
        self.set_prompt()
    else:
        display.display('Please    specify    become    value,    e.g.    `become    yes`')
