Specialize list<string> in config
Summary: Specialize editing, display and validation of list<string> options.
Test Plan: Edited, viewed and validated "environment.append-paths".
Reviewers: codeblock, btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T2255
Differential Revision: https://secure.phabricator.com/D4319