Labels for personalization semantics issues

I set up a few new labels for personalization semantics issues:

  * features - Features that should be supported by personalization
    semantics -
    https://github.com/w3c/personalization-semantics/labels/features
  * implementation - Approaches to add personalization semantics to
    content -
    https://github.com/w3c/personalization-semantics/labels/implementation
  * structure - Ways to structure the vocabulary that impact its
    features -
    https://github.com/w3c/personalization-semantics/labels/structure

I labeled some of the existing issues with these labels, but was using 
hasty judgement about which ones matched. Feel free to adjust the 
labels. I didn't try to tackle all the issues. We might need additional 
labels, there three were just ones that came up in today's call as ones 
we need. Issues can have more than one label.

Michael