module.exports = configure

function configure(base, extension) {
  var index = -1
  var key

  // First do subextensions.
  if (extension.extensions) {
    while (++index < extension.extensions.length) {
      configure(base, extension.extensions[index])
    }
  }

  for (key in extension) {
    if (key === 'extensions') {
      // Empty.
    } else if (key === 'unsafe' || key === 'join') {
      base[key] = base[key].concat(extension[key] || [])
    } else if (key === 'handlers') {
      base[key] = Object.assign(base[key], extension[key] || {})
    } else {
      base.options[key] = extension[key]
    }
  }

  return base
}