Add theme 'dracula'

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

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)

Correct a comment

Call me crazy, but this seems like a typo

Removed incorrect possessive apostrophe

gulp: fixing base path for 'package' task