Add a GitHub action for rustfmt

Checks formatting against the default `cargo fmt` run.

.github/workflows/cpal.yml

@@ -24,6 +24,23 @@ jobs:
         token: ${{ secrets.GITHUB_TOKEN }}
         args: --all --all-features
 
+  rustfmt-check:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - name: Install stable
+      uses: actions-rs/toolchain@v1
+      with:
+        profile: minimal
+        toolchain: stable
+        override: true
+        components: rustfmt
+    - name: Run rustfmt
+      uses: actions-rs/cargo@v1
+      with:
+        command: fmt
+        args: --all --check
+
   cargo-publish:
 
     if: github.event_name == 'push' && github.ref == 'refs/heads/master'