add config test

Configurations.md

@@ -1285,7 +1285,7 @@ fn main() {
 }
 ```
 
-### `true`:
+#### `true`:
 ```rust
 fn main() {
     (foo());

tests/source/configs/remove_nested_parens/remove_nested_parens.rs

@@ -0,0 +1,5 @@
+// rustfmt-remove_nested_parens: true
+
+fn main() {
+    ((((((foo()))))));
+}

tests/target/configs/remove_nested_parens/remove_nested_parens.rs

@@ -0,0 +1,5 @@
+// rustfmt-remove_nested_parens: true
+
+fn main() {
+    (foo());
+}