When you edit package.json, it should trigger a reload of the whole app. (Maybe optionally.)
Previously it closed and reopened the window, but that stopped working, probably at nw@0.13.0.
With chrome.runtime.reload() we should be able to do something even better: reload the whole app!
Some things like chromium-args probably require relaunching the entire process, but there are some snippets of code people have written in nwjs/nw.js#149 to do just that, at least for some platforms.