Allow for terminal colours to be applied to progress characters

I'm trying to colour the progress format characters using the ansi_term crate. Consider the following format:

```[=>_]```

If the equals sign is to be red and bolded, at least on the Windows platform the format string becomes:

```[\u{1b}[1;31m=\u{1b}[0m>_]```

This sort of format string is not compatible with the current library.