Toggle arrow color on click (#1)

* Toggle arrow color on click

When there are a lot of arrows and nodes, it's very hard to follow a single arrow. The stroke-size change on hover helps, but it has two weaknesses:
  1. the arrow still blends with others which are right next to it.
  2. when the diagram is big and the arrow is very large, scrolling will always turn off the hover state.

The color change makes the arrow stand out and it persists through scrolling.

src/Render.php

@@ -104,6 +104,11 @@ public static function html(Graph $graph, array $params = []): string
                      });
                 </script>"
                 : '',
+            '<script>
+                $(document).on("click", "path", e => {
+                    e.currentTarget.style.stroke = e.currentTarget.style.stroke ? "" : "red";
+                });
+            </script>',
             '</body>',
             '</html>',
         ]);