Support "commonmark/markdown" style links

[rpavlik]

That is, like this: [link text](url). Useful for readability, especially when generating nullboard boards from things like gitlab issues/merge requests.

I think the better solution is probably to combine both http link processing regexes into one, instead of negative lookbehind/lookahead, but this was my first attempt. If I get a chance to do better I'll update this or submit a new PR.

I did put a second commit to underline those links, since unlike a string starting with https, it's not clear that these links, once processed, are actually links in the absence of link-like formatting.

[apankrat]

I'd rather hide this behavior behind a config option and have it switched off by default. The reason being is that, currently, clicking on a text does not cause any visual changes, just makes it editable. This patch alters this behavior by making static and editable texts different. Yes, it's doable, there's some utility to it and it's not a big change, but small doable things pave the road to you know where.

[rpavlik]

Sure, that makes sense. It's pretty important for my use case (compensating for cruddy project management abilities in gitlab) but I can see how it's not important to probably many others. I'm fine carrying this in my own fork if you prefer.