Update CDN URLs, add option for local libs and SRI attributes

[kramred]

Hello,

This pull request is a bunch of stuff that is related and introduces two new options:

• --HTMLExporter.use_local_libs=<Bool> (Default: False)
• --HTMLExporter.use_sri_attr=<Bool> (Default: False)

This allows you to create html output where resources are loaded from a local folder and/or hashes for the external resources are added within the tag loading the resource.

This (partially) addresses the following issues: #238 #754 #761 #763

The default behaviour is not changed (except for updated versions of the external resources)

Changes/additions

• Update versions of external resources used (in slides.py still hardcoded)
  • jQuery 2.0.3 → jQuery 3.3.1
  • require.js 2.1.10 → require.js 2.3.6
  • reveal.js 3.5.0 → reveal.js 3.7.0
• Created a template file for (eventually) all external resources urls.tpl in the templates/html folder
  • urls.tpl generates URLs for CDN (as they used to be hardcoded in full.tpl)
  • urls.tpl is imported in full.tpl and depending on the switches the variables are set
  • also generates alternative external URLs (not used yet…)
  • generates URLs for using local resources in the libs/ folder relative to your HTML output file
• Small changes
  • update jinja url in docs

Open issues

• Make options available as switches in the main command, probably in nbconvertapp.py
  • I couldn't get this to work (any help here would be appreciated)

[MSeal]

I need someone more familiar with the js dependencies to take a look at this. Also tests indicate there's some an issue with the jinja template now: jinja2.exceptions.UndefinedError: parameter 'urls' was not provided

[willingc]

Bumping to 6.0 release