new feature: the ability to show line numbers before the text

It can be activated with --linenumbers on the command line or with
'set linenumbers' in a nanorc file, and it can be toggled with M-#.

Signed-off-by: Faissal Bensefia <faissaloo@gmail.com>
Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
11 files changed