Implement styled arguments

[rbrugo]

Hello,
this PR implements a way to apply different styles to different arguments in a single format (or print) call.
It is done by introducing a new function, fmt::styled, which takes an argument of arbitrary type and a color, an emphasis or a text_style, and creates a wrapper around the argument that will be formatted it in the requested style.

A simple example of usage:

void log(std::string_view caller, std::string_view message, bool error = false)
{
    fmt::print("[{}] {}: {}\n",
                fmt::styled(std::chrono::system_clock::now(), fmt::emphasis::bold),
                fmt::styled(caller, error ? fmt::color::red : fmt::color::green),
                message
    );
}

[vitaut]

Thanks for the PR! Overall looks good but please rebase past the CI fix and address inline comments.

[rbrugo]

Everything should be solved now!
Do you think something else needs to be changed?

[vitaut]

Thank you!

[vitaut]

Could you also add it to the docs (https://github.com/fmtlib/fmt/blob/master/doc/api.rst)?

[rbrugo]

Sure!