Support `ned` in `x-dso` and `x-dso-link` tags

[kstar]

Sometimes, an object exists in NED but not in SIMBAD. We want to be able to open NED in those cases. I should be able to specify the database like <x-dso-link ned="NGC 3556:[HK83] 139">knot in M 108</x-dso-link> instead of abusing the simbad attribute.