Implement nested url do API.

[reitermarkus]

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Allows using nested url do blocks in order to allow fetching a chain of URLs for retrieving the final URL.

See Homebrew/homebrew-cask-versions#10620 for examples.

Also, skip auditing URLs using url do unless running with --online.

[BrewTestBot]

Review period will end on 2021-04-01 at 04:17:19 UTC.

[vitorgalvao]

Note: will need update to docs as well.

[BrewTestBot]

Review period ended.