Make `bin/diviner generate` with no arguments mean "generate everything"

Summary: Ref T988. This makes it easier to generate documentation.

Test Plan: Ran with and without `--book`. Examined CLI output.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D8415