fix for na not resetting terminal colors

Hi Brett,

This is my first github pull request, so apologies if I should have made it in a topic branch, by convention.

The problem is that, at least on iTerm2 on the Mac, after you print something in color, *everything* ends up in color.

This was quick and dirty, but I added ANSI escape codes to reset the terminal to its default foreground and background after colors are used.   I've been using na for about ten minutes, so I haven't tested all of its code paths, but the change is straightforward. 


Best regards,
Neil