Add clang-format github action

.github/workflows/format.yml

@@ -0,0 +1,16 @@
+name: Format
+on: [push, pull_request]
+jobs:
+  format:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - name: Install dependencies
+        run: |
+          curl https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+          sudo add-apt-repository -y 'deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-9 main'
+          sudo apt install clang-format-9
+      - name: Format code
+        run: |
+          find src/ -type f -name '*.c' -name '*.c' | xargs clang-format-9 -i
+          git diff-index --quiet HEAD -- || (git diff && exit 1)