config: add optional ‘center’ argument to ‘pad’

When set, the grid contents is centered in the window. I.e. the
left/right and top/bottom margins are equal (+- 1px).

This causes the content to “jump” while doing an interactive resize,
but may still be preferred in e.g. a tiling WM.

Closes #273

config.h

@@ -70,6 +70,7 @@ struct config {
 
     unsigned pad_x;
     unsigned pad_y;
+    bool center;
 
     bool bold_in_bright;
     enum {