Add yaml files to differential.whitespace-matters

Summary: Whitespace has semantic meaning for yaml files, so we shouldn't suppress whitespace-only lines of diff by default.

Test Plan: Edited local config to include yaml files, saw expected whitespace changes.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18775