Add pre- and post-download scripts

[vinnydiehl]

Specially marked scripts in the Gist could be ran and then deleted before/after the files have been downloaded.

This should have a fairly prominent warning. For security I'm marking this as blocked by #1 and in fact maybe this should require a separate flag?

Blocked by #1, #8

[github-actions]

Status: Blocked ❌

Issues blocking this PR:

• Add confirmation #1 ✔️
• Add confirmation #1 ✔️
• Allow files to be made executable  #8 ❌

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This comment was automatically written by the Blocking Issues bot, and this PR will be monitored for further progress.

[vinnydiehl]

The script will be made executable and then ran; I'm marking this as blocked by #8 as the code for chmoding the files should be shared.