bugfix: fix cursor placement on undone auto-indent

When we undo and auto-indent, the cursor should end up at the start of
the line.  It was going to the previous line.

	modified:   editor.rb
feature: undo autoindent whitespace separately from non-whitespace

When autoindent involves non-whitespace characters, record a snapshot
between whitespace and non-ws insertion, so that we can quickly undo
unwanted autoindents.

	modified:   editor.rb