Added some simple colorscheme setters

These don't properly expose the implot style yet in a way that is
consistent with imgui-rs' handling of styles. This is to be added
later.

implot-examples/implot-glium-demo/src/main.rs

@@ -1,4 +1,3 @@
-
 use imgui::{im_str, Condition, Window};
 use implot::Context;