cli : print color scheme info for --print-colors (#3141)

cli : print color scheme info for --print-colors (#3141)

This commit adds a description of the color scheme used in the CLI
when the --print-colors option is enabled.

The motivation for this is that it is not immediately clear what the
color scheme is when using the CLI with the --print-colors option.

Example output:
```console
$ ./build/bin/whisper-cli -f samples/jfk.wav --print-colors
...

main: color scheme: red (low confidence), yellow (medium), green (high confidence)

[00:00:00.000 --> 00:00:11.000]   And so my fellow Americans, ask not what your country can do for you, ask what you can do for your country.
```
The description will not be dispayed if the `--no-prints` options is
set.

Refs: https://github.com/ggml-org/whisper.cpp/issues/3135