Support unicode

[Viatorus]

constexpr auto test = ctp::printf(
  "┌{0:─^{2}}┐\n"
  "│{1: ^{2}}│\n"
  "└{0:─^{2}}┘\n", "", "Hello, world!", 20);

Should print:

┌────────────────────┐
│   Hello, world!    │
└────────────────────┘