Cleanup "plus" command tests for the underscore build.
Add properties used by `_.sortBy` back to the `propWhitelist`.
Add support for `_.camelCase`, `_.endsWith`, `_.escapeRegExp`, `_.kebabCase`, `_.pad`, `_.padLeft`, `_.padRight`, `_.repeat`, `_.snakeCase`, `_.startsWith`, and `_.truncate`.
Adjust ES6 category module builds to comply with ES6 spec.