tools: parse documentation metadata

This commit introduces the Documentation YAML metadata concept to the
documentation.

- Parses added or Added for HTML
- Parses all data for JSON

Ref: https://github.com/nodejs/node/pull/3867
Ref: https://github.com/nodejs/node/issues/3713
Ref: https://github.com/nodejs/node/issues/6470
PR-URL: https://github.com/nodejs/node/pull/6495


Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>