Update node-webkit to 0.10.0 and remove `link` stanza

[node-webkit](https://github.com/rogerwang/node-webkit) has recently reached version 10, but I think we should either remove the `link` stanza, or the cask altogether.

node-webkit is, effectively, a runtime. It works as a binary with which the application developer launches or bundles their code. As such, symlinking it anywhere provides very little utility.