[internals] improve options module
[issue link]Hello,
Since using default export of Options as option initializer, I have a question that is it a good way to use a Class-like naming style to name a function?
I found members in options.js like Modes and defaultOptions are also put in Options now, maybe leave Options as a pure Object, and rename the init function to Options.from(_options) is a better choice?
https://github.com/nuxt/nuxt.js/blob/d7d85d3b4763eacf4a25155c08a01fab3ee5746a/lib/core/nuxt.js#L18