Add continuous integration with Github actions

.github/workflows/build.yml

@@ -0,0 +1,15 @@
+name: CI
+
+on: [push, pull_request]
+
+jobs:
+  test:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macOS-latest, windows-latest]
+    steps:
+    - uses: actions/checkout@v2
+    - uses: iffy/install-nim@v1.1
+    - name: Build
+      run: nimble install -y