Parse cask file for appcast, calc shasum, update

[Amorymeltzer]

See if you like this, tried to match your style here; refs Homebrew/homebrew-cask/issues/15908.

My sed regex on L68 is intentionally vague, in case the appcast is ever downloaded to a file and shasummed that way. A commitment to always pipe through curl would mean it could be changed to sed 's| -$||' if so desired.

Checking to see if the appcast url is still valid is a bit lazy, but should cover the trick; I think it's important to include, though. See also Homebrew/homebrew-cask/pull/15831

[vitorgalvao]

Thank you, I like this.

Will make just a few modifications:

• Make curl silent.
• Move the logic of changing the appcast’s shasum to its own function.
• Use -n to check if the variable is set (I usually do that, as a complement to -z).
• Change sed to awk (consistent to how it’s done to get cask_sha).
• In cask_appcast_url, only check for single quotes (we’ll never have string interpolation there).
• In cask_appcast_url, removed the ,? (since we’ll remove :format anyway). Either way, since we’re only checking for the value inside the quotes, it isn’t necessary.
• Fixed indentation (four spaces → two spaces, and removed a tab).
• Whitespace around pipes.

[vitorgalvao]

Merged in 68a0f7b.