[core] Add sanity exec command that registers part loader + babel

[rexxars]

This adds a new command to the Sanity CLI: exec.
Basically, if you have a studio and have some scripts associated with it that you want to run, it would be nice to be able to use the parts system as you would otherwise, and preferably also be able to use ES6-features.

This is now possible with sanity exec <script.js>. It registers babel with the es2015-node4 and stage-2 presets, and registers the Sanity plugin loader.

For instance, you can now make a script like this:

import client from 'part:@sanity/base/client'

const products = await client.fetch('*[_type == "product"]')
console.log(products)

[bjoerge]

WANT! (and LGTM)