Add 2 new config opts, fix one scrot command and redirect nohup to /d…

[laur89]

…ev/null

• add 2 new notification messages configuration options:
  • enable_notify_messages & notify_level;
• update the help documentation regarding -s, -f & -w options; those were missing;
• redirect nohup output to /dev/null not to leave garbage behind;
• (for linux) - change notify-send message TTL from 500 to 3000 ms;
• (for linux) - fix selected window capture scrot command (was missing -u flag);

[laur89]

Note that the config page needs to be updated regarding enable_notify_messages & notify_level options if this pull request is accepted.