Added numerical positional selection.

When pressing 1-9, the nth menu item will be selected. This behaviour is
unavailable when searching and is intended for quick access when the
actual position is already known.
History editing now *replaces* the active element.

Previously, editing the history would simply plunk a new entry into the
history. This is unexpected. This change has a weird interaction with
the history being enabled/disabled, since the edited value will be
removed. This isn't unexpected, so just don't do it.
Updated readme with new known issue.
Disable selecting action items by index.