Rewrite specification

* Update to the new style of unist
  <syntax-tree/unist@8055bf4>
* Rename `Text` > `Literal`, `TextNode` > `Text` (syntax-tree/unist#17)
* Reword definitions
* Add introduction, references, links to terms
* Add common mixins for shared fields
* Move enumerations to their own section
* Add description of content models (GH-21)
* Remove marking of YAML as non-standard

Related to syntax-tree/unist#18
Related to syntax-tree/unist#17
Closes GH-21.

Author: wooorm