Adding "table of contents" to posts

[cjxe]

I forked this repo and added "table of contents" feature for posts.

It looks like the following image:

Would anyone be interested in this feature? If so, I am happy to make a PR and add it.

[uxodb]

you should have it float to the left/right and make it sticky maybe, that'd be nicer i think.

[keivanipchihagh]

Please do @cjxe! It's a great feature.