IE / Edge SVG rendering fix (chanind#31)

* Adding geometry helper functions

* adding a usePolygonMasks option which masks without using strokes for crappy IE

* using clip path for polygon masking instead of alpha masks because MS makes bad browsers

* adding check to strip out extra parallel points in medians

* final tweaks to rendering and refactoring

* fixing final rendering glitch by fixing clipping path overlaps and adding a circle element to the clip tip

.eslintrc

@@ -5,7 +5,7 @@
     "no-for-of-loops"
   ],
   "rules": {
-    "id-length": [2, {"exceptions": ["x", "y"]}],
+    "id-length": [2, {"exceptions": ["x", "y", "i"]}],
     "no-unused-vars": [2, {"vars": "all", "args": "none"}],
     "max-len":[0],
     "no-loop-func": [0],

demo/index.html

@@ -28,7 +28,7 @@ <h1 class="title">Hanzi Writer</h1>
 		</div>
 
 		<script src="https://cdn.polyfill.io/v2/polyfill.min.js"></script>
-		<script type="application/javascript" src="../dist/hanzi-writer.min.js"></script>
+		<script type="application/javascript" src="../dist/hanzi-writer.js"></script>
 		<script type="application/javascript" src="test.js"></script>
 
 	</body>