add download.retry field

[andykais]

schema:

interface RetryInterface {
  limit?: number // defaults to zero
  retryOnStatusCodes: number[] // defaults to [202, 503, 429, 504]
  delay?: number // defaults to zero
}
type Retry = number | RetryInterface
interface DownloadConfigInit {
  ...
  retry: Retry | undefined
}

Investigate putting logic inside and outside observable.

investigate possibility of retrying page based solely on parsed content. E.g. if page has title "Too Many Retries" then retry same as regular retry (note would vastly change the api and should be thought about in a broader sense)