Add markdown linting (#2)

Reviewed-on: https://git.tomaka.onl/atomaka/atomaka.com/pulls/2

.drone.yml

@@ -2,6 +2,11 @@ name: test
 kind: pipeline
 
 steps:
+  - name: linting
+    image: python
+    commands:
+      - pip install -r requirements.txt
+      - mdformat --check _posts/
   - name: jekyll-build
     image: ruby
     commands: