Drawing on top of slide

[blobule]

Hi! Thanks for this great work. I am using it for my teaching slides.

I absolutely need to draw on top of my slides when I present them, so I added some code to do it.
I am not submitting a pull request because the code is small, and if you like it it will be faster for you to integrate it nicely. It relies on the fabric.js (https://github.com/fabricjs) library.
Here is an example from my course (its in french:-):
http://www.iro.umontreal.ca/~roys/ift1005/01-internet.html
When in full mode, here are some keys to try:

• a enters drawing mode
• b exits drawing mode
• just click and drag to draw. All drawings "stick" to the current slide.
• z to erase current slide
• 1, 2, 3 to change color
• 8, 9,0 to change pen shape

You can see this in action in my recorded course :

• https://classe.iro.umontreal.ca/w/wiCDvwCTHXdTkqo3GNNGSr?start=52m43s
• https://classe.iro.umontreal.ca/w/8u3Wg9mwVePkJ9E21ZnDvr?start=51m08s

The code that needs to be added to a slide is this:

    <!-- live drawing -->
    <link rel="stylesheet" href="shower/dessin.css">
    <div id="canvas-wrapper"> <canvas id="sheet"></canvas> </div>
    <script src="js/fabric.js"></script>
    <script src="shower/dessin.js"></script>

I have this on gitlab, but its all in the slides I provided. Feel free to use it as you see fit.
What's missing: drawing everything in list mode, and printing everything correctly.

Thanks again for this great project,
Sébastien