Add pre-commit git hook to check code style

The hook automatically fixes most code style mistakes
and should be installed once with:

```sh
ln -s ../../bin/pre-commit .git/hooks/pre-commit
```

bin/pre-commit

@@ -0,0 +1,33 @@
+#!/bin/bash
+
+pass=true
+
+files=$(git diff --cached --name-only --diff-filter=ACMR | grep -E '\.(php|phtml)$')
+if [ "$files" != "" ]; then
+
+    # Run php syntax check before commit
+    while read -r file; do
+        php -l "$file"
+        if [ $? -ne 0 ]; then
+            pass=false
+        fi
+    done <<< "$files"
+
+    # Run php-cs-fixer validation before commit
+    echo "$files" | xargs ./vendor/bin/php-cs-fixer fix --diff --config .php_cs.dist
+    if [ $? -ne 0 ]; then
+        pass=false
+    fi
+
+    # Automatically add files that may have been fixed by php-cs-fixer
+    echo "$files" | xargs git add
+fi
+
+if $pass; then
+    exit 0
+else
+    echo ""
+    echo "PRE-COMMIT HOOK FAILED:"
+    echo "Code style validation failed. Please fix errors and try committing again."
+    exit 1
+fi