code: def install_plugin(self, dir, entry_script=None):
          
          self.runtimepath.append(dir)
          if entry_script is not None:
              self.command('runtime! {0}'.format(entry_script), False)
docs: Install *Vim* plugin.
  
          :param string dir: the root directory contains *Vim* script
          :param string entry_script: path to the initializing script