Add the tokenizer.ondemand setting to Core for now.

Summary:
T2255 lists it as "???" and we don't have a "Misc" category and it seems silly
to make it for one option, so stick this in core for now.

Test Plan: Went to the setting page and saw the bool options.

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4389