Make it possible to use a javascript-based page-level toc

Uses the https://tscanlin.github.io/tocbot/ library to build the sidebar toc
if the following is set in your config.toml file:

[params.jstoc]
enable = true

Note that by default, h2-h4 headings are included in the toc. To change that,
provide a comma-separated list of headings like this:

[params.jstoc]
enable = true
custom_headings = "h2, h3"
custom_headings = "h2, h3"

Compared to the default sidebar toc, this solution has the following benefits:

- a marker shows the current location of the screen (useful for long pages)
- shows the correct title even if the title contains a shortcode
- shows the title in the toc even if it was included from another file

Author: fekete-robert