Recommend ext-filter

Author: GrahamCampbell

composer.json

@@ -21,8 +21,9 @@
         "symfony/polyfill-ctype": "^1.9"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8.35 || ^5.0 || ^6.0 || ^7.0",
-        "bamarni/composer-bin-plugin": "^1.3"
+        "ext-filter": "*",
+        "bamarni/composer-bin-plugin": "^1.3",
+        "phpunit/phpunit": "^4.8.35 || ^5.0 || ^6.0 || ^7.0"
     },
     "autoload": {
         "psr-4": {
@@ -34,6 +35,9 @@
             "Dotenv\\Tests\\": "tests/Dotenv/"
         }
     },
+    "suggest": {
+        "ext-filter": "Required to use the boolean validator."
+    },
     "extra": {
         "branch-alias": {
             "dev-master": "4.1-dev"