it's time to set TERM to xterm-256color
256: have a config to choose whether to set TERM to xterm-256color
add -2/--256color option to set TERM to xterm-256color

Man page was modified accordingly.
256: fix color cube
compat: fix backspace

According to
http://invisible-island.net/ncurses/terminfo.ti.html#tic-xterm-basic ,
xterm sends ^H == \b == 0x08.