Add support for std::complex

[dcblack]

Since std::literals allows for std::complex literals, it seems natural that fmt would support them.

#include <complex>
#include "fmt/format.h"

int main() {
  using namespace std::literals;
  auto i1 = 1.0i;
  auto i2 = 2.0if;
  auto i3 = 3.0il;
  auto s = fmt::format( "{} {} {}", i1, i2, i3 ); //< missing formatters for complex numbers
  assert( "1.0i 2.0i 3.0i" == s );
}

[vitaut]

Thanks for the suggestion. A PR to add a formatter for complex to fmt/std.h would be welcome.