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 think the `pub` isn't necessary for a macro to leave a module. All that is necessary is the `#[macro_escape]` attribute, [example from the std lib](https://github.com/rust-lang/rust/blob/9ae48286a99627391e30da3888217e6050230cfe/src/libstd/macros.rs#L39).