Support $mod+Return for starting up a terminal

This comes with the default configuration on i3. I removed this KBD originally
because I was hoping to use the $mod+{,Shift+}t KBDs frequently. I still do, but
as I get more comfortable with i3, splits, parent containers, etc. I may prefer
to create terminals this way. We'll see...

configs/shared/misc/.config/i3/config.shared

@@ -51,6 +51,8 @@ bindsym $mod+Ctrl+Shift+e      split h; exec emacsclient --create-frame ~/progra
 bindsym $mod+Ctrl+Shift+c      split h; exec google-chrome
 bindsym $mod+Ctrl+Shift+s      split h; exec spotify
 
+bindsym $mod+Return exec st
+
 # pass integration
 bindsym $mod+Ctrl+backslash exec ~/.config/i3/passmenu # mirroring the 1password KBD