use ANSI-escapes instead of backspace sequences

Backscape sequences don't work well (if at all?) with UTF-8 multi-byte characters. Besides that, using ANSI-escapes is simpler and takes less bytes, possibly is even easier to draw too. As extra, we can emit colour codes to match HTML input to some extent. MUAs such as Mutt support these sequences, see http://www.mutt.org/doc/manual/#tab-ansi-esc