Gulp livereload: include subfolders to watch for changes in html and md

Add theme 'dracula'

New theme using color palette from [Dracula](https://draculatheme.com/)
dark editor theme available for quite a number of different
applications.

Currently it's only watching for changes to `.html` and `.md` files located in the root of the project. I was frustrated when livereload stopped working for me: turns out it was because I put my content into subfolders.

fix: truncated long r-fit-text on pdf

Fix typo in plugin.js

withing -> within

Co-authored-by: Giacomo Zinetti <giacomo.zinetti@giko.it>

fix issue where auto-animate did not work when jumping three or more slides (i.e. longer than the view distance)

fix: pdf page numbering for fragment group

always use css transforms for presentation scaling (zoom has too many quirks even if it is slightly sharper on ldpi displays)