Optional libraries

[kenohassler]

This PR (closes #19):

• replaces JQuery code with native JS,
• makes loading (external) JS configurable,
• updates mermaid, katex and mapbox,
• adds a small improvement to dark mode (still, only colour inversion).

In contrast to what @martinmcwhorter proposed, I'm not inlining the initialization code. I decided so in order to benefit introduction of Content Security Policies.

[RatanShreshtha]

Just add new config option in docs and README as well

[kenohassler]

I'll add a commit later this evening. Thanks for the feedback!

[RatanShreshtha]

Thanks @kenohassler