Allow Nuance commands to try to apply immediately

Summary:
Ref T12738. By default, we process Nuance commands in the background. The intent is to let the user continue working at full speed if Twitter or GitHub (or whatever) is being a little slow.

Some commands don't do anything heavy and can be processed in the foreground. Let commands choose to try foreground execution.

Test Plan: Threw complaints in the trash, saw them immediately go into the trash.

Reviewers: chad

Reviewed By: chad

Subscribers: avivey

Maniphest Tasks: T12738

Differential Revision: https://secure.phabricator.com/D18015