Beautify help messages

[QuarticCat]

Before:

After:

If you add AppSettings::ColoredHelp, the output can be better:

However, currently this cannot work with your --no-colors flag. See
clap-rs/clap#1880 and clap-rs/clap#1185.

Actually, this not only affects help messages, but also affects error output of clap. (BTW, your current error handling is not consistent with clap's built-in. If you want a more consistent error output, you can check clap::Error. But that will also introduce some problems.)

One compromise is to use NO_COLOR environment variable to disable colors. If you like that, I can commit relative codes.

[QuarticCat]

I would like to say that --no-colors is not a good design. Usually we prefer automatically determine outputting colors or not. And add a flag --color=always|auto|never to control that. Although this is also hard to implement for clap's built-in color output.

[bootandy]

That's cool I didn't know you could do that.