thumbnails preview stayed the same when playing next video, by changing video url and setting up vtt thumbnails

[urosZoretic]

fix: thumbnails preview stayed the same when playing next video, by changing video url and settingu up vtt thumbnails

when source url in video.js player is changed and player already exists. calling vttThumbnails({...}) setup again will cause plugin to create multiple thumbnail-preview div containers. As consequence plugin can end up creating multiple containers for different video url sources, but it will show only first one created..

suggested fix in this PR: #15
With this fix I have resolved issued to firstly remove old one if it exists and then plugin can create new one as normal. This resolve issue when playing next video in a sequence by changing video url and setting up thumbnails again, but thumbnails stayed the same