Colors from rustc are not shown on Windows.

Environment is Windows 7 with `cmd.exe`. Rust and cargo are stable 1.42, nightly does the same.

Here’s a screenshot:
![Screenshot](https://user-images.githubusercontent.com/3986214/78836576-ab3a6980-79f2-11ea-9bd6-f66a87d91d53.png)
```rust
struct S;
fn main() {
    let x = S;
    drop(x);
    drop(x);
}
```