Parameterize the common ngrams threshold

Summary:
Ref T13000. Since other changes have generally made the ngrams table manageable, I'm not planning to enable common ngrams by default at this time.

Instead, make the threshold configurable with "--threshold" so we can guide installs through tuning this if they want (e.g. PHI110), and tune hosted instances.

(This might eventually become automatic, but just smoothing this bit off for now feels reasonable to me.)

Test Plan: Ran with --reset, and with various invalid and valid --threshold arguments.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13000

Differential Revision: https://secure.phabricator.com/D18710