add an option to show image in fullscreen

[zhangzq]

Is your feature request related to a problem? Please describe. 目前，显示图片时，图片会比屏幕小一点（目测约90%的屏幕大小）。我希望图片能真正做到全屏。

Describe the solution you'd like 增加一个选项，使得图片显示时为100%。

[fengyuanchen]

可以尝试在图片显示后调用 zoomTo 方法改变图片大小。

[zoum79]

hello ! I think it is possible to show titles (alt) when showing images in fullscreen. But how ??? A second wish : I would like to show the tooltip at the opening of images, not only when zooming. I thank you very much if you can give me solutions. zoum79

[fengyuanchen]

See #526.