message: add a `strict` mode to readMetadata()
  Test cases will set this to `false`, because they're
  missing `scripts.start` and other required properties in this function
diff: @@ -16,6 +16,7 @@ module.exports = async function (path, {
  deploymentType = 'npm',
  deploymentName,
  quiet = false,
  + strict = true,
  isStatic = false
  }) {
  let pkg = {}
  @@ -50,7 +51,7 @@ module.exports = async function (path, {
  }
  }
  - if (!pkg.scripts || (!pkg.scripts.start && !pkg.scripts['now-start'])) {
  + if (strict && (!pkg.scripts || (!pkg.scripts.start && !pkg.scripts['now-start']))) {
  const e = Error('Missing `start` (or `now-start`) script in `package.json`. ' +
  'See: https://docs.npmjs.com/cli/start.')
  e.userError = true
  @@ -81,7 +82,7 @@ module.exports = async function (path, {
  throw e
  }
  - if (docker.length <= 0) {
  + if (strict && docker.length <= 0) {
  const e = Error('No commands found in `Dockerfile`')
  e.userError = true
  throw e
  