Issue title: slides with fixed height and variable width per each slide
 Issue body: I'm trying to make horizontal slider that contains images of various widths that should "stitch" together seamlessly.

So far everything works great, except that the slider tries to re-size all my slides to a certain width set in  `$SlideWidth`.

My question is how do I get the image to either have a set width for each slide (inline styles?) or do an auto width depending on the ration and height of the image?

 Comments: 
 Comment 0: with u="image" attribute specified, jssor slider will fill your image in each slide according to $FillMode.

To let image keep original size (or specify size manually), please remove u="image" attribute.

 Comment 1: I am having this problem too!

Wait, so you're saying if I remove `u="image"`, the width will become variable?

 Comment 2: Yes. With no u="image" image will keep original size (or the size specified by css manually).

 Comment 3: I'm encountering something weird now. My image is keeping its size, but the surrounding DIV is defaulting to width of 100px

{'source': 'GithubIssues', 'repo_name_with_owner': 'jssor/slider', 'issue_id': 47030795, 'repo_stars': 2063.0}