Fixes#6138
Naturally this does not work for many other editors/aliases,
but it's still nice that we can make it work for some common
editors without requiring any configuration.
Of course this approach is not terribly flexible; but it's
alwyas possible to just wrap edit_command_buffer and set an
EDITOR that knows about the cursor position. It doesn't
feel important enough to add a configuration option.
This implements a standard function and bindings for editing the command
line in an external editor. This feature has been requested multiple
times in the past year with various solutions cut and pasted into those
issues. This change combines the best aspects of those solutions.
Fixes#1215
