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