Add option to change plot color theme

[Seanny123]

I'm going to add an option to change the color scheme of multi-line plots, because our six-color limit is annoying me.

I'll leave the Nengo colour function as the default, but I'm going to add the options that you can see in this example. I'm just going to hook it up to the context menu because I can't think of any clever user-interface or nice automatic way of doing that. That being said, I'm obviously open to suggestions and feedback.

Mildly related to #400 and #430.