New feature: Page Up/Down keys jump in history (start, end).

Note: Also recognizes the Insert now as well, but doesn't do anything
with it.  This may be used in some future extension.  A possibility
would be to have an additional overwrite mode, and use this key to
toggle between the two.
New feature: Provide a public accessor to the getWindowSize() functionality.
The accessor returns only the number of columns, i.e. window width.