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

foot.ini

@@ -8,7 +8,7 @@
 # initial-window-size-pixels=700x500  # Or,
 # initial-window-size-chars=<COLSxROWS>
 # initial-window-mode=windowed
-# pad=2x2
+# pad=2x2                             # optionally append 'center'
 # shell=$SHELL (if set, otherwise user's default shell from /etc/passwd)
 # term=foot
 # login-shell=no