Add Github Actions

The tests do spew a few warnings but no errors so this should pass. I
did not bother adding macOS/Windows to the test matrix as there's
probably nothing platform-dependant in this repo, but it could be done
on request.

.github/workflows/ci.yml

@@ -0,0 +1,20 @@
+name: ci
+on: [push, pull_request]
+
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest]
+        node: ['10', '12', '14']
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: ${{ matrix.node }}
+      - run: npm install
+      - run: npm test