Floating images / div in slides

[DavidLeoni]

Define html tag data-jupman-style and during build rename it to style, so images or divs can be freely positioned in the result without going wild while editing in Jupyter

• question: mainly meant for slides, but could it make sense in other contexts, too?
• note this can be a hacky workaround for some cases of #140

Example:

<img src="img/astropi-humidity-histogram.png" 
     data-jupman-style="position: absolute;
                        right: 1vw;
                        top: 15vh;
                        width: 47%;" >