added support to select a color-scheme. added schemes 'xterm' and 'xterm...

The colors in the HTML didnt seem right to me :)

After a research (http://en.wikipedia.org/wiki/ANSI_escape_code#Colors),
it seems ansi2html uses a scheme very close to VGA.

I added an option to select schemes and added "xterm" and "xterm-bright" schemes.