Feature: Add target attribute for navigation link

[xepozz]

This PR adds target attribute for links in the navigation menu.

P.S. I think may be add attributes property for more flexible configuration these link?

[github-actions]

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[xjlin0]

in the meantime, I'd use [link name](https://www.example.com/){:target="_blank"}

[iBug]

This one is sufficiently small for inclusion, but when the next attribute comes up (e.g. if someone asks for rel="whatever", I'll be eager to deprecate everything except url and title (which is in fact link text and thus essential) and move all of them to an attributes property.