add Github Action for code styling

.github/workflows/php-cs-fixer.yml

@@ -0,0 +1,29 @@
+name: Check & fix styling
+
+on: [push]
+
+jobs:
+    style:
+        runs-on: ubuntu-latest
+
+        steps:
+            -   name: Checkout code
+                uses: actions/checkout@v2
+
+            -   name: Fix style
+                uses: docker://oskarstark/php-cs-fixer-ga
+                with:
+                    args: --config=.php_cs --allow-risky=yes
+
+            -   name: Extract branch name
+                shell: bash
+                run: echo "##[set-output name=branch;]$(echo ${GITHUB_REF#refs/heads/})"
+                id: extract_branch
+
+            -   name: Commit changes
+                uses: stefanzweifel/[email protected]
+                with:
+                    commit_message: Fix styling
+                    branch: ${{ steps.extract_branch.outputs.branch }}
+                env:
+                    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}