Add configuration of shorthand link, e.g. GitHub linking

[ghost]

It'd be really cool if you could write like [ {something} ](boostio/boostnote#1491) to link to the github repository without writing out the whole URL. Even maybe like gh: before to define it fully?

i.e. Check out this [Feature Request](gh:boostio/boostnote#1491) to compile to Check out this Feature Request

[tatoosh]

It would be nice to set the url. So i can use my selfhosted gitlab as issue link.

[ghost]

Good thinking, @tatoosh! Maybe we could have an option in preferences to define a URL for the linking? You could set it to like git.itspugle.ga, and it'll hyperlink to git.itspugle.ga/username/repository/issues/123 etc?

Maybe you can set up placeholders, incase it doesn't follow that convention?