[Feature Request] Support http URLs in Link cards

[tupini07]

First of all, thanks for making masterplan I've been enjoying it a lot :) I use it more for general organization of stuff than for making games but it's still a great fit.

Anyway, I was thinking that right now the "link" card type has two possible link mode s link to card and link to file, and it would be great if we could somehow also have it link to a URL since it's very common that one wants to link to some external content without actually having to pull it into the ".plan" project.

I would imagine that when "visiting" such a link card it would go ahead and open the URL in the default browser.