Add support for light colors.

This adds support for "light" foreground colors and "light" background colors.  On many terminals, the map to slightly different colors that "bold", but this is the best we can do on Windows.

Note: I added another parameter to `winterm.fore()` and `winterm.back()` to say you want the light version of the color.  Hopefully, this doesn't cause any backwards-compatibility issues.

Also, I saw that one of the tests was erroring due to using None as a stream, instead of a Mock object.  That's been fixed too.