Change the way Vim highlights groups with reversed colors

Mixing `cterm` with `ctermfg` or `ctermbg` doesn't work on some terminals, the `cterm` attribute is ignored (so the colors aren't reversed). Switching the fg and bg color and removing the `cterm` attribute should result in the same highlighting, but supporting all terminals.
Merge pull request #59 from georgehemmings/fix-reverse-colors

Change the way Vim highlights groups with reversed colors