Even More Customizable Straight Beam for LEGO Technic

This is a remix of Steve Medwin's awesome [Customizable Straight Beam for LEGO Technic](http://www.thingiverse.com/thing:630626).

I added the ability to make it half-height or to have "plus"-shaped holes and cleaned up the code a bit. With these modifications you can make pieces that fit on the plus-shaped axles. To specify the number and shape of the holes in Customizer, use a string with +'s and o's. For example, the string "+ooo+" for a block with five holes: pluses at the ends and thre