refs #167: add support to link check

Using [markdown-link-validator](https://github.com/jenkin/markdown-link-validator/tree/feature/sparkfabrik-enhancements) from github repo.

Makefile

@@ -11,6 +11,9 @@ cli:
 logs:
 	docker-compose logs -f
 
+check:
+	docker-compose run --rm documentation npm run check
+
 # Sparkkit based themes specific commands.
 theme-watch:
 	chmod +x bin/npm

custom/package.json

@@ -5,12 +5,14 @@
     "author": "Paolo Pustorino <paolo.pustorino@sparkfabrik.com>",
     "main": "server.js",
     "scripts": {
-      "start": "node server.js",
-      "test": "echo \"Error: no test specified\" && exit 1"
+        "start": "node server.js",
+        "check": "markdown-link-validator -q -e -o -i ^/downloads/ -i drupal.org -i udemy.com -i sparkfabrik.loc -i acme.sparkfabrik.com ./content",
+        "test": "echo \"Error: no test specified\" && exit 1"
     },
     "license": "https://creativecommons.org/licenses/by-sa/4.0/",
     "dependencies": {
-      "raneto": "0.17.3",
-      "gulp": "4.0.2"
+        "gulp": "4.0.2",
+        "markdown-link-validator": "github:jenkin/markdown-link-validator#feature/sparkfabrik-enhancements",
+        "raneto": "0.17.3"
     }
-  }
+}