create "_data/tutorial_tags.yml" and populate with tags

See [maptime.github.io's list of tags](https://github.com/maptime/maptime.github.io/blob/master/_data/tutorial-tags.yml) for examples of how we can tag posts.

@namessanti this could be cool with your idea about having a series on experimental maps.  Keeping a "experimental" tag would let us identify this branch of projects.
