Add a build+test github action, as a quick and cheerful CI.

Signed-Off-By: David Anderson <dave@natulte.net>

.github/workflows/linux.yml

@@ -0,0 +1,48 @@
+name: Linux
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - '*'
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    if: "!contains(github.event.head_commit.message, '[ci skip]')"
+
+    steps:
+
+    - name: Set up Go 1.13
+      uses: actions/setup-go@v1
+      with:
+        go-version: 1.13
+      id: go
+
+    - name: Check out code into the Go module directory
+      uses: actions/checkout@v1
+
+    - name: Basic build
+      run: go build ./cmd/...
+
+    - name: Test build
+      run: go test ./...
+
+    # TODO: set up slack notification.
+    # - uses: k0kubun/action-slack@v2.0.0
+    #   with:
+    #     payload: |
+    #       {
+    #         "attachments": [{
+    #           "text": "${{ job.status }}: ${{ github.workflow }} <https://github.com/${{ github.repository }}/commit/${{ github.sha }}/checks|${{ env.COMMIT_DATE }} #${{ env.COMMIT_NUMBER_OF_DAY }}> " +
+    #                   "(<https://github.com/${{ github.repository }}/commit/${{ github.sha }}|" + "${{ github.sha }}".substring(0, 10) + ">) " +
+    #                   "of ${{ github.repository }}@" + "${{ github.ref }}".split('/').reverse()[0] + " by ${{ github.event.head_commit.committer.name }}",
+    #           "color": "danger"
+    #         }]
+    #       }
+    #   env:
+    #     SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
+    #   if: failure() && github.event_name == 'push'