Add backwards compatible support for Promise based api

[ggarber]

Tested with this code:

applescript
  .execString(script)
  .then(rtn => {
    if (Array.isArray(rtn) && rtn.length > 0) {
      console.log('Currently selected tracks in "iTunes":');
      rtn.forEach(function(songName) {
        console.log("\t" + songName);
      });
    } else {
      console.log('No tracks are selected in "iTunes"...');
    }
  })
  .catch(err => {
    console.log(err);
  });

It will enable to use node-applescript with modern async/await code.