Add corresponding `highlighted` properties for `shadowColor`, `shadowOffset` and `shadowRadius`

These additions are [not in line with the documentation](https://github.com/mattt/TTTAttributedLabel/issues/168), but the existing `shadowRadius` property is not either so I'm not sure if this is an appropriate pull request. 

The existing behavior is that if the label is `highlighted` to not add any shadow. This contradicts regular `NSString` drawing behavior, which is to use the same `shadowOffset` and `shadowColor` whether highlighted or not.