replaced /Library/Updates/index.plist check with softwareupdate --list

[mpanighetti]

• added separate $MSG_UPDATING alert while updates are running in the background (deferred update during restart not working in 10.13+ #4)
  • gives user option to manually run updates (dynamically describes method of manual updates depending on macOS version)
• added preflight softwareupdate --list (more accurate than reading from /Library/Updates/index.plist, with tradeoff of longer script runtime #3) before caching updates (script exits if --list returns no recommended results)
• removed all /Library/Updates dependencies as that path is now under SIP in macOS 10.14+
• set softwareupdate --download to only grab --recommended updates (as per README and the intent of the script to only trigger for security updates requiring restart)
• renamed trigger_updates_at_restart to check_for_updates, moved recon/unload to dedicated exit_without_updating function
• added explicit 10.13 and 10.14 support in version checks
• moved updater mechanism to dedicated run_updates function
• renamed clean_up_before_restart function to clean_up (since it isn't always run as part of a restart action)
• changed App Store icon path (.png resource no longer exists in macOS 10.14, and jamfHelper natively supports .icns files)
• iterated version in script and build-info.plist to 2.0
• updated Casper references to Jamf Pro in README
• updated example security patches in README to more recent builds (matching naming convention from Apple security updates)

[mpanighetti]

New commit brings pretty much all of the changes I've made to the fork I use for regular update patching. Addresses issue of 10.14 not allowing deletion of contents of /Library/Updates (yay SIP!) and brings general 10.13/10.14 compatibility.

[homebysix]

Excellent set of additions! Works well in my limited testing. Thank you!