Don't initialize id-map when rendering md files

Adding these "known" values to the table of used ids is only required
when embedding markdown into a rustdoc  html page and may yield
unexpected results when rendering a standalone `*.md` file.

src/librustdoc/markdown.rs

@@ -83,7 +83,7 @@ pub fn render(input: &str, mut output: PathBuf, matches: &getopts::Matches,
     }
     let title = metadata[0];
 
-    reset_ids();
+    reset_ids(false);
 
     let rendered = if include_toc {
         format!("{}", MarkdownWithToc(text))