add a line number argument to open_editor

Salut Jonas!

This is the v2 of #118. I figured out that vi accepts (as many other editors do) the +num argument *before* the filename. So I just add it in the editor_argv, if needed.

So far, I tested vi, vim, emacs, nano, gedit, geany and they are known to work fine.

The option is applied on the diff and stage view.

Cheers,
Vivien