Add markdown-link-check test on Travis CI

This would help make sure all links alive and not dated.

.travis.yml

@@ -22,6 +22,17 @@ stages:
 
 jobs:
   include:
+    - stage: Test
+      env:
+        - TEST: Markdown link check
+      language: node_js
+      node_js:
+        - lts/*
+      install:
+        - npm i -g markdown-link-check
+      script:
+        - find . -name "*.md" -exec markdown-link-check {} \;
+
     - stage: Test
       env:
         - TEST: Editor Config check

travis.yml.template

@@ -20,6 +20,17 @@ stages:
 
 jobs:
   include:
+    - stage: Test
+      env:
+        - TEST: Markdown link check
+      language: node_js
+      node_js:
+        - lts/*
+      install:
+        - npm i -g markdown-link-check
+      script:
+        - find . -name "*.md" -exec markdown-link-check {} \;
+
     - stage: Test
       env:
         - TEST: Editor Config check