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