cleanup: simplifying buffer history class

plus, trying to implement multiple saved buffer states.
So far, so good; but really needs testing.  Especially for memory
issues.

	modified:   TODO.md
	modified:   editor.rb
feature: multiple "saved" states for the buffer history.

Now doing "revert to saved" multiple times, takes you past the
current saved state to previous saved states.  Good for bumping
through recent changes.

Needs more testing.

	modified:   editor.rb