Allow Diviner groups to be configured in `.book` files

Summary:
Ref T988. Currently, every class/function needs to be annotated with `@group`, but 99% of this data can be inferred from file structure, at least in this project. Allow group specifications like:

  "paste" : {
    "name" : "Paste",
    "include" : "(^src/applications/paste/)"
  }

..to automatically put everything defined there in the "paste" group. A list of regexps is also supported. Depends on D6855.

Test Plan: Regenerated documentation with `bin/diviner generate --book src/docs/book/phabricator.book --clean`, observed all Paste stuff go in the paste group.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D6856