Add experimental emphasis support

[Rakete1111]

This patch adds support for text emphasis: bold and underline.

#define FMT_HEADER_ONLY
#include "include/fmt/color.h"
#include "include/fmt/format.h"

int main() {
  fmt::print(fmt::emphasis::bold, "bold\n");
  fmt::print(fmt::emphasis::underline, "underline\n");

  fmt::print(fmt::emphasis::bold, fmt::color::red, "bold and red\n");
  fmt::print(fmt::emphasis::underline, fmt::color::red, fmt::color::yellow,
             "underline, red and yellow\n");
}

[foonathan]

I think the number of parameters here isn't going to scale well.

I'd suggest creating a single formatting options struct that can be created from a single formatting enum or maybe something like this fmt::emphasis::bold | fmt::color::red, but @vitaut decides.

[Rakete1111]

I was thinking about that, but then I decided against it, because I don't know how to differentiate between foreground and background colors. fg(fmt::color::red)? But that's not nice to look at IMO. Do you have any idea?

[foonathan]

Right, good catch.

I'm actually not too opposed to something like fg(fmt::color::red).

[vitaut]

I agree with @foonathan that we should have a single formatting style struct to keep the number of overloads manageable. Using fg(color) and bg(color) to differentiate seems fine. I like it better than the current positional API with arbitrary order.

[vitaut]

And thanks for the PR! This is a cool feature.

[Rakete1111]

So, I introduced a new class text_format, which stores the text formatting:

fmt::print(fmt::emphasis::bold | fg(fmt::color::red), "Hello in bold and red\n");

fmt::text_format hackerman = bg(fmt::color::black) | fg(fmt::color::green);
fmt::print(hackerman | fmt::emphasis::bold, "imma hack u\n");

[vitaut]

Looks great. Just a few smallish comments.

[vitaut]

All looks good, only gcc 4.4 looks unhappy: https://travis-ci.org/fmtlib/fmt/jobs/464344769

[vitaut]

Merged, thanks!

[Rakete1111]

@vitaut Can you retroactively add a default constructor for text_style that I completely forgot? Thanks! :)

[vitaut]

Can you retroactively add a default constructor for text_style that I completely forgot?

Sure, added.