[Feature Request] `ProgressStyle`: enable/disable colorized output based on draw target

[tonywu6]

Hello! This is a follow up to #86:

It seems that the default implementation draws to stderr, but whether the output is colorized is dependent on whether stdout is a tty, so if you do something like foo > log, you get progress bars, but they're uncolored.

... which is a bit surprising for programs that intend their stdout to be piped but also use stderr for progress reporting. Right now I'm manually doing a set_colors_enabled(colors_enabled_stderr()) to convince indicatif to print colors.

I'm wondering if you'd be interested in a PR to make it configurable for ProgressStyle, now that console has features like for_stderr for differentiating color support on stdout vs. stderr.

How I imagine this to work:

1. Have a ProgressStyle::for_stderr() that explicitly sets color detection for its TemplateParts; this could be a public API or just pub(crate);
2. Within ProgressBar::set_style(), implicitly call ProgressStyle::for_stderr() based on whether the bar's ProgressDrawTarget is TermTarget::Stderr (I'm not sure if doing this makes sense for other types of target though)

Thank you for this crate!

[djc]

Sounds okay to me, open a PR?

[tonywu6]

Opened #699