Headings in rustdoc markdown chunks should have anchors & with copyable links

[huonw]

E.g. the section documenting the {..., select, ...} formatting specifier is buried deep inside the doc page: http://static.rust-lang.org/doc/master/std/fmt/index.html

It would be nice if the headings had anchors (with links nearby, so accessing the appropriate anchor for a link was easy); the above could be, say, http://static.rust-lang.org/doc/master/std/fmt/index.html#sec.select and it would jump straight to the relevant part.