Implement playpen defaults

Avoids issues where we enable `editable` but forget to specify `editor`
(eg, https://github.com/rust-lang/rust-by-example/issues/963).  Since we
already include Ace editor, seems like we can just treat it as the
default.

src/config.rs

@@ -274,12 +274,22 @@ pub struct HtmlConfig {
 }
 
 /// Configuration for tweaking how the the HTML renderer handles the playpen.
-#[derive(Debug, Clone, Default, PartialEq, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(default, rename_all = "kebab-case")]
 pub struct Playpen {
     pub editor: PathBuf,
     pub editable: bool,
 }
 
+impl Default for Playpen {
+    fn default() -> Playpen {
+        Playpen {
+            editor: PathBuf::from("ace"),
+            editable: false,
+        }
+    }
+}
+
 
 #[cfg(test)]
 mod tests {