Add share style link, copy button

This adds a copy to clipboard button and input with the style parameter
pre-populated after exporting a style to an anonymous gist. Also
includes the URL as an input next to the button.

src/styles/_export.scss

@@ -3,3 +3,15 @@
     display: inline-block;
   }
 }
+
+.maputnik-render-gist {
+  p {
+    margin: 10px 0;
+  }
+
+  input.maputnik-string {
+    margin-left: 5px;
+    width: 60%;
+    display: inline-block;
+  }
+}