Add 'remark' and plugin to verify internal links in markdown

package.json

@@ -22,7 +22,8 @@
     },
     "scripts": {
         "build": "gulp build",
-        "lint": "eslint src/js",
+        "lint": "eslint src/js && npm run-script remark",
+        "remark": "remark -f --use 'validate-links=repository:\"sampotts/plyr\"' '{,!(node_modules),.?**/}*.md'",
         "test": "echo \"Error: no test specified\" && exit 1"
     },
     "devDependencies": {
@@ -58,6 +59,8 @@
         "postcss-custom-properties": "^7.0.0",
         "prettier-eslint": "^8.8.2",
         "prettier-stylelint": "^0.4.2",
+        "remark-cli": "^5.0.0",
+        "remark-validate-links": "^7.0.0",
         "rollup-plugin-babel": "^3.0.5",
         "rollup-plugin-commonjs": "^9.1.3",
         "rollup-plugin-node-resolve": "^3.3.0",