Add eraser option

[mjclarke94]

When trying to see the effect of small changes it would be useful to be able to erase points after they've been drawn.

[koaning]

Oh! I like that idea and will keep this issue open as a reminder.

For the short term we also offer a way to undo previously drawn items, but I can see how it's annoying to redo something from 5 edits ago unless there is an eraser of some sorts.