anchor tracking implementation

[eyllanesc]

• Implement anchor tracking: When navigating using the scrollbar then the url is updated based on the toc.
• Fix that a toc element is active if the bottom header is above the content div.

In the following images it is shown in the urlbar that changes while the scrollbar is scrolling and coincides with the active item of the toc. Also there is only active item if the bottom of the header is on the content div.

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[HEIGE-PCloud]

http://localhost:1313/basic-markdown-syntax/

When there is content before the first title of a post and the bottom of the header is on that piece of content, the last item in toc is activated.

[eyllanesc]

http://localhost:1313/basic-markdown-syntax/

When there is content before the first title of a post and the bottom of the header is on that piece of content, the last item in toc is activated.

Okay, it seems that my calculations assume that the space before the first toc anchor is small so it fails in the post you indicate, later I fix it and upload the patch.