Add an option to remove `target="_blank"` from all links

[samglover]

Building on #56, it might be nice to remove target="_blank" (and rel="noopener, noreferral") from all links.

[samglover]

Along with this, it might also be a good idea to remove the checkbox on the link dialog for opening a link in a new tab.

[samglover]

And why not also have the option to remove target=_blank from local links only.