Add vocabulary files and keyword dependencies.

[handrews]

This just covers the Core spec, the rest will be done
as a follow-on. Fixes #996.

Please read these instructions before commenting!

The following topics are in bounds for PR comments:

• Does this follow as expected from issue Minimalist vocabulary file for expressing keyword dependencies. #996
• Is it clear?
• Is it complete?
• Are there whole sections, paragraphs or sentences that should be moved or just dropped?

For smaller-scale sentence tweaking or wording preferences, please make notes in #847 but I don't want to get into that here. If it's clear enough to be understood, that's good enough for now.

The following topics are out of bounds for comments on this PR, but if you can build consensus on slack or wherever and come back here with something that has consensus behind it, I will gladly switch to the consensus view. I just don't want to debate alternatives myself (for reasons explained in detail on slack). If an alternative emerges with broad support, as long as I don't see a major problem I'll just accept it:

• Other things that could be added
• Different names for things
• Completely different approaches

[karenetheridge]

I see this was closed.. but let's revisit next year for the next draft, as there are lots of gems in here that I don't want to lose.

[gregsdennis]

@handrews wanted to do this for draft 07, but we never got to a place where we could define WHAT a keyword was. There's another issue (#911) floating around (I think @ssilverman or @jdesrosiers opened it) that goes into some ideas around metaschemas and vocabs that Henry summarily dismissed.

[Relequestual]

Ok. Open another issue and reference this PR and that issue or PR and we will see what we can do! =D

[gregsdennis]

Definitely not for this draft. Needs more discussion.