Allow user to customise table of contents maximum index with front matter

[mrtolkien]

Current code:

minimal-mistakes/_layouts/single.html

Line 39 in f15e02e

{% include toc.html sanitize=true html=content h_min=1 h_max=6 class="toc__menu" %}

As you can see, h_min=1 h_max=6 are hard coded.

It would be nice to have a syntax similar to jekyll-toc, with something like:

toc_min_level: 1
toc_max_level: 6

I am not issuing a pull request as I don’t know much about Ruby, Liquid, and the project guidelines, but this looks like a nice feature to add.