Added new color for 'Z' state.

Just like pull request #137, I now added the same color for zombie processes.
I'm not sure whether I should have created a new color `PROCESS_Z_STATE` or if both would share the same format, and opted to make both states highlight the same way.

It currently looks like this:

![2014-10-18-024708_138x152_scrot](https://cloud.githubusercontent.com/assets/887736/4688165/dee086aa-5669-11e4-9ff6-a222041186fc.png)


(I was actually making a patch to allow zombie processes to appear even when kernel threads were hidden, but then I found out Debian Squeeze's htop is from 2008 and that bug has since been fixed.)