b56e63e2a9 removed this because it was
deprecated and not clearly useful. This commit adds this operation back
to string lists and keyvalue lists, but with one important change. It
operates via the actual values and not indexes. So you can use
--foo-del=bar,bar2 to remove bar and bar2 from foo. The difference from
using -remove is that this is subject to escaping rules and has the same
caveats as -add. Note that -del wasn't added back to the object settings
list because you can already remove multiple items with -remove from it.
Done with 4a084c0df8. The reasoning was
that it was "confusing", but without using -add it is impossible to
append multiple items to a list in a single command and just overall
makes this less powerful. The code works fine. You might find yourself
in escaping hell, but that's on the user to deal with.
Also it's worth noting that -remove with object settings lists can
actually remove multiple items and unlike the other list option types.
