convert _.extend to PHET_CORE/merge in non-runnable repos, and update doc usages, https://github.com/phetsims/phet-core/issues/71
convert _.extend to PHET_CORE/merge in non-runnable repos, and update doc usages, https://github.com/phetsims/phet-core/issues/71

phetsims/scenery-phet,phetsims/scenery-phet,phetsims/scenery-phet