Added both the build() and the begin()+end() options for Plot

- Copied the trick from imgui-rs to notice if a begin() is left
  unmatched
- Changed plot_line to be a struct, but might go away from that
  again. Initial idea was to add things like setting markers and
  colors to this as struct methods, but it seems this will lead
  to lots of code overhead compared to the pushing and popping
  stylevars approach taken in the C++ implementation.

testexample/src/main.rs

@@ -1,5 +1,5 @@
 use imgui::*;
-use implot;
+use implot::{Plot, PlotLine};
 
 mod support;
 
@@ -21,17 +21,17 @@ fn main() {
                 ui.checkbox(im_str!("Show demo"), &mut showing_demo);
 
                 // Draw a plot
-                implot::Plot::new("Demo plot")
+                Plot::new("Demo plot")
                     .size(400.0, 300.0)
                     .x_label("awesome x label")
                     .y_label("awesome y label")
                     .build(|| {
-                        implot::plot_line(&vec![2.0, 2.0], &vec![2.0, 1.0], "Left eye");
-                        implot::plot_line(&vec![4.0, 4.0], &vec![2.0, 1.0], "Right eye");
+                        PlotLine::new("Left eye").plot(&vec![2.0, 2.0], &vec![2.0, 1.0]);
+                        PlotLine::new("Right eye").plot(&vec![4.0, 4.0], &vec![2.0, 1.0]);
 
                         let x_values = vec![1.0, 2.0, 4.0, 5.0];
                         let y_values = vec![1.0, 0.0, 0.0, 1.0];
-                        implot::plot_line(&x_values, &y_values, "Mouth");
+                        PlotLine::new("Mouth").plot(&x_values, &y_values);
                     });
             });