Allow "bin/bulk export" to merge multiple queries and accept more flexible flags

Summary:
Ref T13210. Minor usability improvements to "bin/bulk export":

• Allow --class task to work (previously, only --class ManiphestTaskSearchEngine worked).
• If you run --query jXIlzQyOYHPU, don't require --class, since the query identifies the class on its own.
• Allow users to call --query A --query B --query C and get a union of all results.

Test Plan:

• Ran --class task, --query A --query B, --query X (with no --class), got good results.
• Ran various flavors of bad combinations (queries from different engines, invalid engines, query and class differing, ambiguous/invalid --class name) and got sensible errors.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13210

Differential Revision: https://secure.phabricator.com/D19738