improve link shortening & add [descriptive](https://links)

[cblgh]

this pr fixes shortening of already short http links (i.e. stops appending of .. to short links)

it also makes it possible to link passages of text using [markdown style](https://guides.github.com/features/mastering-markdown/#examples)

i'm doing this as a pr since it's been a subjective while since i last contributed, and the previous time i made a patch with regexes turned out, ehm, sub-optimal.

potential concerns! the (url) regex isn't strict at all, so you can basically [link](whatever) i'm not sure how much of a problem that will pose, i don't think it should allow for executing javascript since we escape the html before we even get to this stage

anyway i think this feature will be great for The Daily Descent among other things :~

[neauoire]

I'll merge this for now, but I would prefer if we used the curlies {text|link} since we're already using curlies for {%image.jpg%} and {bold} and {italic}.

[cblgh]

aight changed ^