Unstabilise unstable_features

cc #1974

Author: nrc

Configurations.md

@@ -2062,7 +2062,7 @@ Enable unstable features on stable channel.
 
 - **Default value**: `false`
 - **Possible values**: `true`, `false`
-- **Stable**: Yes
+- **Stable**: No
 
 ## `license_template_path`
 

src/config/mod.rs

@@ -123,7 +123,7 @@ create_config! {
         "What Color option to use when none is supplied: Always, Never, Auto";
     required_version: String, env!("CARGO_PKG_VERSION").to_owned(), false,
         "Require a specific version of rustfmt.";
-    unstable_features: bool, false, true,
+    unstable_features: bool, false, false,
             "Enables unstable features. Only available on nightly channel";
     disable_all_formatting: bool, false, false, "Don't reformat anything";
     skip_children: bool, false, false, "Don't reformat out of line modules";