doc_file

Move your Rust crate's documentation into external files.

Why?

This is a proof of concept implementation of [rust-lang/rust issue

15470][issue]. I hope to iron out questions and issues out of tree and then

contribute this back to rustc itself.

Example usage

```rust

![feature(phase)]

// Paths are relative to the source file.

![docfile = "examplecrate.markdown"]

[phase(plugin)] extern crate doc_file;

[docfile = "complicatedthing.markdown"]

pub struct ComplicatedThing; ```

Open issues

The syntax #[doc_file = "..."] is a little ugly. Ideally it would be something like #[doc(file = "...")].