Table sorting support

**Is your feature request related to a problem? Please describe.**
I make often realtively huge tables. I would like to make them all in HackMD, but so far I cannot sort them and to ssort by hand is hard.

**Describe the solution you'd like**
The best solution would be, if some kind of sorting could happen, e.g. by clicking on an arrow next to a header to sort alphabetically (https://github.com/Mottie/GitHub-userscripts/wiki/GitHub-sort-content is a good source to see how I would like it to behave). It wouldn't necessarily be persistent (change anything in the source).

**Additional context**