aws-sns-cli: update checksum
audit: Improve url audits.

* Add url check for GitHub home pages using http.
* Add url check for GitHub repo urls using http.
* Include url in error messages about url issues.

Closes #18372.
audit github downloads

We want the new-style "/archive/" downloads.
ack: use archive download
zsh-completions: use archive download
yeti: use archive download
shocco: use a tarball
New terminal-notifier formula (revised first version)

The `terminal-notifier` utility is analogous to Growl's `growlnotify` CLI tool. 

To use the program as distributed, one must call the inner binary of an app bundle (due to a library-linking quirk the author notes, in https://github.com/alloy/terminal-notifier/blob/master/README.markdown) – this formula sidesteps this awkward mode of execution with an exec script (see L#15).