Reduce built package size

[cderv]

❯ checking for hidden files and directories ... NOTE
  Found the following hidden files and directories:
    inst/reveal.js-4.1.2/.npmignore
    inst/reveal.js-4.1.2/.github
  These were most likely included in error. See section ‘Package
  structure’ in the ‘Writing R Extensions’ manual.

❯ checking installed package size ... NOTE
    installed size is  6.3Mb
    sub-directories of 1Mb or more:
      reveal.js-4.1.2   6.1Mb

I think we need to do that before CRAN release