Add support for custom colors and sizes; adjust default size

[CWSpear]

I wanted to be able to set custom colors (and opacities) and also, I noticed the icons are a lot larger than the MDI ones. I set a default size of 24px so they are more in line with the MDI ones, but the size is customizable as well.

I also added a note about sub-directories in custom icons. I wasn't sure if that would work and I didn't want to put all the icons in the same folder, since that'd require renaming, etc. Turns out, sub-directories do work (I mean, I see why now that I look at the code), so I figured I'd add it to the docs!