Add example infrastructure

* Add "Shared" projects for all examples
* Update "Colors" example with better TrueColor demo
* Use same namespace for all examples

examples/Shared/Extensions/ColorExtensions.cs

@@ -0,0 +1,15 @@
+namespace Spectre.Console.Examples
+{
+    public static class ColorExtensions
+    {
+        public static Color GetInvertedColor(this Color color)
+        {
+            return GetLuminance(color) < 140 ? Color.White : Color.Black;
+        }
+
+        private static float GetLuminance(this Color color)
+        {
+            return (float)((0.2126 * color.R) + (0.7152 * color.G) + (0.0722 * color.B));
+        }
+    }
+}