README example: load MathJax over https

tiny followup to #1103

README: suggest decktape as an alternative for PDF rendering

It works around the current PDF rendering bugs like #1261

Allows to specify PORT from env on multiplex plugin

enabling launching presentation with npm-start

broken <table> html in demo deck

`licenses` field is deprecated

the `licenses` field in package.json is deprecated. https://docs.npmjs.com/files/package.json#license

The `license` field is the recommended field and expects an SPDX expression.

PDF exporting also works with Chromium

Added missing comma on the configuration example

Correct Readme.md -> Theming -> sky

For those who can't install Google Chrome (e.g. work network policies) but can use Chromium, Chromium does work for creating the pdf. I updated the readme accordingly. Thanks!

Theming/sky.css has dark gray text on light blue background; not white text

Make npm dependency usable by Browserify through npm