Add support for multiple HDD destinations

[XCHSystems]

I have not found anything that would allow this apart from doing this the old way by specifying a plot count on an SSD and having something like plow.py move the plots to multiple disks.

It would be a more simplified option to be able to specify multiple disks in say a config file, and the plotter will round robin to each disk

[rkroelin]

Plow seems like the simpler answer if you have multiple plotters. You only need to configure the list of destinations in one place instead of on each plotter.