Merge branch 'build-stage' into 'master'

Add build stage

See merge request !13

.gitlab-ci.yml

 default:
   image: node:12
-  before_script:
-  - npm install
+
+build:
+  stage: build
+  script: npm install
+  artifacts:
+    paths:
+      - node_modules
 
 test:
   script: npm test