logging: try to avoid using ANSI color codes when not logging to a terminal

Color on the console is nice, but it is annoying to get these codes when for
example redirecting to a file.

The formatter doesn't know exactly where the message will be used - it is fully
configurable. In the default configuration, the messages are passed to a
StreamHandler to sys.stderr .

As an approximate optmization, hardcode the color formatters to only emit color
formatting if logging to something that isatty.