rustdoc: Submit examples to play.rust-lang.org

This grows a new option inside of rustdoc to add the ability to submit examples
to an external website. If the `--markdown-playground-url` command line option
or crate doc attribute `html_playground_url` is present, then examples will have
a button on hover to submit the code to the playground specified.

This commit enables submission of example code to play.rust-lang.org. The code
submitted is that which is tested by rustdoc, not necessarily the exact code
shown in the example.

Closes #14654

src/librustdoc/lib.rs

@@ -126,7 +126,9 @@ pub fn opts() -> Vec<getopts::OptGroup> {
         optmulti("", "markdown-after-content",
                  "files to include inline between the content and </body> of a rendered \
                  Markdown file",
-                 "FILES")
+                 "FILES"),
+        optopt("", "markdown-playground-url",
+               "URL to send code snippets to", "URL")
     )
 }