Keep colors in compiler error messages

[mgeisler]

The playground currently removes the colors from the compiler error messages. Using the example from Comprehensive Rust 🦀, we get

in the playground, but

The red, blue, and green colors have been lost.

The colorful and helpful error messages from the compiler are a big selling point of Rust, so it would be great if we could keep them in the playground as well.

This was first reported in google/comprehensive-rust#407, but it looks like an upstream issue to me. It is related to #879 in the sense that it might require some client-side changes.