Add colorization switch

[fedarko]

So the user could selectively color by G/C content, depth, or ostensibly any other number of features.

On the UI side this could really just be a series of radio buttons ("none", "by G/C content", "by depth"), combined with a tool to pick the extreme colors accordingly (maybe just two input boxes for two hex colors).

Programmatically, adjusting this stuff shouldn't be that hard. The most difficult part will likely be algorithmically defining the gradients between two arbitrary colors, since I forget how I did that between red and blue (...but it shouldn't be too hard to figure it out).

Also, we should disable the radio button(s) for which the assembly graph has no data -- if no colorization fields are given, potentially just disable the entire selection area.

[fedarko]

for now it should be alright to just have two radio buttons -- "none" and "G/C content". should be simple to implement. would changes take effect upon redrawing the graph? either way, just be explicit.