Add new diagnostic writer using annotate-snippet library

This adds a new diagnostic writer `AnnotateRsEmitterWriter` that uses
the [`annotate-snippet`][as] library to print out the human readable
diagnostics.

The goal is to eventually switch over to using the library instead of
maintaining our own diagnostics output.

This commit does *not* add all the required features to the new
diagnostics writer. It is only meant as a starting point so that other
people can contribute as well.

[as]: https://github.com/rust-lang/annotate-snippets-rs

src/librustc_errors/lib.rs

@@ -33,6 +33,7 @@ use termcolor::{ColorSpec, Color};
 mod diagnostic;
 mod diagnostic_builder;
 pub mod emitter;
+pub mod annotate_rs_emitter;
 mod snippet;
 pub mod registry;
 mod styled_buffer;