Make diviner documentation generation book-oriented

Summary:
I want to allow a single project to generate multiple "books" of documentation, so we can separate user-facing documentation from technical documentation and such.

Generalize the ".divinerconfig" file into a "diviner book" configuration file.

Since only the "generate" workflow actually reads any of this stuff, move it all down into the generate workflow.

Also, namespace the cache.

Test Plan: Ran bin/diviner generate --book src/docs/user.book, saw appropriate output. Verified cache generated in a namespace in .divinercache/.

Reviewers: btrahan, indiefan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D4857