feat: rule test supports function

src/Rule.js

@@ -87,7 +87,11 @@ const Rule = Orderable(
       }
 
       if (!omit.includes('test') && 'test' in obj) {
-        this.test(obj.test instanceof RegExp ? obj.test : new RegExp(obj.test));
+        this.test(
+          obj.test instanceof RegExp || typeof obj.test === 'function'
+            ? obj.test
+            : new RegExp(obj.test),
+        );
       }
 
       return super.merge(obj, [

test/Rule.js

@@ -131,6 +131,15 @@ test('toConfig with values', t => {
   });
 });
 
+test('toConfig with test function', t => {
+  const rule = new Rule();
+  const test = s => s.includes('.js');
+
+  rule.test(test);
+
+  t.deepEqual(rule.toConfig(), { test });
+});
+
 test('merge empty', t => {
   const rule = new Rule();
   const obj = {