add color error/warn highlighting

Highlights the 'E', 'W' and 'P' of error, warning and progress output messagesrespectively with color+bold (red for error, yellow for warn, no specific color for progress).

The default depends upon whether stdout and stderr are both detected to be ttys, true if they are, false otherwise, obviously to help avoid the terminal codes entering output redirected to a log file or otherwise piped. There are --color and --no-color controls giving user choice and which ignore fd type.

Obviously the purpose is improved user experience. I came to this from noticing the lack of color in live-build output when an error occurred.

I've done light testing of basic sudo debootstrap sid . usage only. I have not tried using the test suite.

I spent more time than I expected on it trying to follow all of the redirection of FDs to be confident the terminal codes only get used for ttys (unless forced); I'm reasonably confident, but this aspect could do with particular focus from a reviewer more familiar with the codebase than I.