CLI: add mechanism to pass callbacks as plugin options through metalsmith.json about metalsmith HOT 1 OPEN

JSON.parse(jsonText, async (key, value) => {
  if (value.startsWith('@from:')) {
    const importPath = path.resolve(metalsmith.directory(), value.slice('@from:'.length))
    const resolved = await import(importPath)
    return resolved.default
  }
  return value
})

const imports = []
const config = JSON.parse(jsonText, (key, value) => {
  if (value.startsWith('@from:')) {
    const importPath = path.resolve(metalsmith.directory(), value.slice('@from:'.length))
    imports.push(import(importPath).then(imported => {
      return {
        imported: imported.default,
        keypath: getFullKeyPathSomehow()
      }
    }))
  }
  return value
})

Promise.all(imports).then(({ keypath, imported }) => {
  _.set(config, keypath, imported)
})