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.

Look at the progress bar formatting code, likely in a module handling format strings. The issue shows an example with ANSI escape sequences. You need to modify the parsing to allow escape codes within the format string without breaking the progress character counting. Check how the format string is currently parsed and rendered, and ensure the escape sequences are stripped for length calculations but passed through for output. Test on Windows to confirm the colored characters display correctly.

Scritto dal modello di indicizzazione a partire dal testo della issue.