Table of Contents support

[mendax1234]

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In the official minimal mistakes repo, there is a table of contents function which is integrated in the single layout. But in the academicspage fork, this function is missing. Enabling the toc function can make blog posts more clear.

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Add jekyell-toc plugin. Try to refer to what minimal mistakes has done already. Although I think there are much to change.

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I haven't thought about the alternative since this function is not available on academicpages now.

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