Allow external html in rustdoc for crates.

Updated documentation to reflect md->html.
Modularized external file loading.
auto merge of #15237 : zzmp/rust/feat/markdown-in-crate-documentation, r=huonw

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).