Add a "noplaypen" class for rust code samples.

This class will supress the "play" button in the html backend (which you
can also do with the "ignore" class), but it will still let the code be
tested via `mdbook test` (which is not possible with the "ignore" class).

This is useful for code examples that don't really do much (and so the
user doesn't gain much from running them), but as an author you still
want to test them to guard against syntax errors and typos and the like.

Author: eminence

src/renderer/html_handlebars/hbs_renderer.rs

@@ -580,7 +580,9 @@ fn add_playpen_pre(html: &str, playpen_config: &Playpen) -> String {
             let classes = &caps[2];
             let code = &caps[3];
 
-            if (classes.contains("language-rust") && !classes.contains("ignore"))
+            if (classes.contains("language-rust")
+                && !classes.contains("ignore")
+                && !classes.contains("noplaypen"))
                 || classes.contains("mdbook-runnable")
             {
                 // wrap the contents in an external pre block