feat: add config-option for fragment-index-divider (#20)

test/fixtures/valid/existing-heading-fragment-divider/awesome.md

@@ -0,0 +1,13 @@
+# Awesome
+
+## Existing Heading
+
+### Repeated Heading
+
+Text
+
+### Repeated Heading
+
+Text
+
+### Repeated Heading

test/fixtures/valid/existing-heading-fragment-divider/existing-heading-fragment.md

@@ -0,0 +1,9 @@
+# Valid
+
+[Link fragment](./awesome.md#existing-heading)
+
+[Link fragment Repeated 0](./awesome.md#repeated-heading)
+
+[Link fragment Repeated 1](./awesome.md#repeated-heading_1)
+
+[Link fragment Repeated 2](./awesome.md#repeated-heading_2)

test/index.test.js

@@ -239,6 +239,16 @@ test("ensure the rule validates correctly", async (t) => {
           },
         },
       },
+      {
+        name: "should be valid with multiple existing element id fragments",
+        fixturePath:
+          "test/fixtures/valid/existing-heading-fragment-divider/existing-heading-fragment.md",
+        config: {
+          "relative-links": {
+            "fragment-index-divider": "_",
+          },
+        },
+      },
       {
         name: "should ignore external image links",
         fixturePath: "test/fixtures/valid/ignore-external-image.md",