Diagnostic reporting support for the codespan crate.

To get an idea of what the colored CLI output looks like with codespan-reporting, clone the repository and run the following:

cargo run -p codespan-reporting --example=emit
cargo run -p codespan-reporting --example=emit -- --color never

You should see something like the following in your terminal:

We're still working on the output - stay tuned for updates!