[dwm] [dvtm] scrollback and 256color support

Hi, the following two patches add support for a scrollback buffer and
enhanced color in dvtm.

scrollback:
        - Simply set up some keybinding to scrollback(), passing in "-1" or
"1" to scroll up or down.

256color:
        - Allows the use of nice VIM color schemes
        - Ncurses doesn't allow more than 256 fg/bg color pairs on screen at
the same time.
        - 8 or 256 colors only. If your terminal supports 88 colors, you
need to set up your own variant of rxvt, e.g. rxvt-88color