I already bought Steam4C2, but it's not compatible with the latest version of nw.js and Steam SDK. It actually uses v0.18.5, but v0.18.7, v0.19.0 and v0.19.2 are already out, and there's no support for those versions. You also need to use Steam dlls from SDK 138 instead of SDK 139, which doesn't sound that cool. I'm not saying that the plugin is bad in anyway, it seems to work really well for some people, but i alway rather have native features.
BTW, i saw your game on the store, it's awesome! Would love if you could check out mine too.
http://store.steampowered.com/app/516480/
Sorry if i said something wrong, it's not my main language.
The 1.39 SDK update didn't come out that long ago, and doesn't bring any new gameplay-related features, so that's kind of irrelevant in terms of support. While it would be nice to have support for EVERY version of nwjs, that's somewhat unrealistic considering how often they're released and what's involved in supporting them due to C2's SDK requiring separate builds of the plugin for each version of nwjs that needs maintenance. Things take time. The Steam4C2 plugin is already head & shoulders above any other Steam-related support - as in, it actually works, across many versions of nwjs - and hopefully it'll get updated soon.
Also, the existing Steam4C2 version for nwjs 19.x will work in 19.2, even though it's not listed as working in that version.