Add github action

Author: m-o-e

.github/workflows/crystal.yml

@@ -0,0 +1,18 @@
+name: Crystal CI
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    container:
+      image: crystallang/crystal
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install dependencies
+      run: shards install
+    - name: Run tests
+      run: make test