Collapsible TOC list entries on configurable level

[asperling]

You can easily test the changes by using the docker image designed for developing bookdown themes:

docker run -it \
    -v $(pwd):/app \
    -v /path/to/your/clone/of/bookdown-bootswatch-templates/templates:/templates \
    -p 8080:8080 \
    -e TOC_SUBLIST_LEVEL=4 
    asperling/bookdown-template-develop \
    docs/bookdown.json

It will generate your documentation, make it available at http://localhost:8080 and regenerate it if you work on the template files,

[tobiju]

First of all great work @asperling. But I found a bug. If you use export TOC_SUBLIST_LEVEL=1 I get a strange Processing content with errors. Could you please fix that error and I will accept your pull request.

[asperling]

Done.