Added rudimentary way to customize position lists.

One can now specify custom position lists/commands in the config
file. For example, one can change the "top" positions by adding

	[commands]
	top =	table 2x1 (1,1)
		table 3x1 (1,1)
		table 3x1 (1:2,1)
		table 2x3 (1,2)

which will make a list with

	1) the top half of the screen
	2) the top third of the screen
	3) the top two thirds of the screen
	4) the top half, center third of the screen