black is defined wrong in exporter colors

[DerKorb]

When exporting dxf, the default color is black and is defined as 0 in core/exporter.ts 0 is an invalid value for autocad and it will fail to open the file for it. The right black value is 255.

[danmarshall]

Would you mind creating a PR with this change? Also, helps to add a link to documentation in the code :)