Add command to make an item an instance/subclass

[nichtich]

wdtaxonomy --isa Qparent Qchild
wdtaxonomy --broader Qparent Qchild

Both should check whether a instance-of/subclass-of relation exists to skip or modify (the latter requires maxlath/wikibase-edit#2). An item should never be both instance and subclass of the same other item.

[nichtich]

Should better be done with wikidata-cli but a howto would be nice:

1. query taxonomy
2. spot instances wrongly classified as subclasses
3. somehow pipe to wikidata-cli