Add story tags attribute

[klembot]

This is a forthcoming change in Twine 2.4. An early form of this functionality has landed on the develop branch if you'd like to take a look, but keep in mind that the code is in pre-alpha state and could easily trash any existing stories/cause weird issues if you downgrade to 2.3.

[greyelf]

I'm curious about the initial purposed of this new story level tag.

[tmedwards]

For author tags—as discussed in today's committee meeting.

Presumably for things like genre and content tags, as one example.

[mcdemarco]

Sounds good.