Boxplot element and category coordinate have been added.

Squashed commit of the following:
    Added testing notes.
    Added the test coverage for the changed code.
    Added documentation.
    TextAlignment enum has been added to be used as a hint for the rendering.
    Category coord has been added.
    Added explicit Quartiles struct.
    Initial Boxplot element has been added.

Author: nuald

examples/wasm-demo/www/package.json

@@ -16,7 +16,7 @@
     "wasm",
     "rust",
     "webpack",
-	"visualization"
+    "visualization"
   ],
   "author": "Plotters Developers",
   "license": "MIT",