Add a line number argument to open_editor

This patch allows opening the editor from the diff or stage view, at the
actual line number.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>

Increase the editor argument array size.

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>
Blob view: open editor at selected line
Stage view: open untracked file at selected line
Detect errors and allow editor line number to be disabled

First of all a big warning is shown when the editor reports an error.
Secondly, the line line number feature is disabled.
tigrc(5): document show-line-numbers