feature: even in 'col' mode, single-line means effectively 'row' mode.

If mark and cursor are on the same line in 'col' mode, let it
behave as row mode.  This is convenient for cutting and pasting
parts of lines without having to switch out of 'col' mode.

	modified:   editor.rb
cleanup: simplified ask method a little

Created a class to handle the ask_history.

	modified:   editor.rb