Someone requested some features, but I've forgotten who! So, whoever you
are, please let me know if this works for you...
Rectangular selection can now be extended via the keyboard by mapping keys
to the new commands Selector Extend Up, Selector Extend Down, Selector
Extend Left and Selector Extend Right. These commands rectangularly extend
the selection in the specified direction.
Currently, I can't figure out how/if keyboard listening is supported for an
editor. Thus, the Selector selections don't delete when you press a key. So,
I've added Selector Delete and Selector Backspace commands which can be
mapped to Delete and Backspace. It's not a great solution, but it's all I
can think of.