Allow html in crate docs.

This makes the `in-header`, `markdown-before-content`, and `markdown-after-content` options available to `rustdoc` when generating documentation for any crate.

Before, these options were only available when creating documentation *from* markdown. Now, they are available when generating documentation from source.

This also updates the `rustdoc -h` output to reflect these changes. It does not update the `man rustdoc` page, nor does it update the documentation in [the `rustdoc` manual](http://doc.rust-lang.org/rustdoc.html).
I don't like these wrappings. Someone may wish to insert HTML to wrap the whole body, that is, `before_content` with something like `<div class="real-rustdoc-content">` and `after_content` like `</div>`. If they do want `section`s themselves, the user can just have such things in the files they include.