add pkg for building Spoticord exe

[xaoseric]

Adds pkg dependency, and a build script for building a exe.

[xaoseric]

@Phineas this allows you to run npm run build on windows, and it outputs a .exe that you can use instead of having to do npm install, so you could in theory do a release. It also supports linux and mac os https://github.com/zeit/pkg

[davidcralph]

sounds good