Support for intersection and union

[freb]

Currently only a single argument or input is permitted, either via stdin or arg. We should support arbitrarily many inputs (stdin, plug any number of args) and be able to perform intersection and union on them.

Union is basically already supported and is what you get when you just join two parameters with a comma. But intersection would be helpful for comparing lists.

Complement could also be helpful, where you are looking for the elements in one set that are not in another set.