Merge pull request #2248 from alexheretic/master

Fix config warnings leaking into stdout

src/config.rs

@@ -361,7 +361,7 @@ macro_rules! create_config {
                             self.$i.1 = true;
                             self.$i.2 = val;
                         } else {
-                            println!("Warning: can't set `{} = {:?}`, unstable features are only \
+                            eprintln!("Warning: can't set `{} = {:?}`, unstable features are only \
                                       available in nightly channel.", stringify!($i), val);
                         }
                     }