start using GitHub Actions as CI

The tests are disabled for now, since they're not fully functional yet.
But we can at least ensure the code builds, passes vet, and follows
gofmt.

.github/workflows/test.yml

@@ -0,0 +1,26 @@
+on: [push, pull_request]
+name: Test
+jobs:
+  test:
+    strategy:
+      matrix:
+        go-version: [1.12.x, 1.13.x]
+        platform: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.platform }}
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v1
+      with:
+        go-version: ${{ matrix.go-version }}
+    - name: Checkout code
+      uses: actions/checkout@v1
+
+    # TODO: reenable once the tests are finished
+    # - name: Test
+    #   run: go test ./...
+
+    - name: gofmt check
+      run: diff <(echo -n) <(gofmt -d .)
+      if: matrix.platform == 'ubuntu-latest'
+    - name: vet check
+      run: go vet ./...