Add global channel selectors, plot each channel separately.

This allows effects to be applied to specific channels, instead of all
channels like before.

The syntax for selectors is as follows:
	[[start]-[end][,]...]
Examples:
	-          all
	2-         2 to n
	-4         0 through 4
	1,3        1 and 3
	1-4,7,9-   1 through 4, 7, and 9 to n
Update README.md.