Make string path shorthands consistent and opt-in

[jgonggrijp]

Following up on #231, jashkenas/underscore#2877 and #220 (stage 4):

• Turn the internal keysFromPath function into a public function that can optionally be used to override the new _.toPath function in the upcoming core Underscore 1.12.
• In all functions that work with paths, use the new _.get from core Underscore 1.12.
• Update the documentation to reflect that path behavior is configurable from a single point of truth.