create continuous integration (#40)

automatically run npm test for each pull request and commit

.github/workflows/unit_tests.yml

@@ -0,0 +1,15 @@
+name: Unit Tests
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - run: npm ci
+    - run: npm test