在參數中加入playlist這個參數，若只有一個影片就填一個就好
<iframe class="embed-responsive-item"id="ytplayer" type="text/html" width="640" height="360" src="https://www.youtube.com/embed/M7lc1UVf-VE?&autoplay=1&loop=1&rel=0&showinfo=0&color=white&iv_load_policy=3&playlist=M7lc1UVf-VE"
frameborder="0" allowfullscreen></iframe>出處：http://stackoverflow.com/questions/25779966/youtube-iframe-loop-doesnt-work
官方參數文件(中文)：https://developers.google.com/youtube/player_parameters?hl=zh-cn#playlist
