Helper functions

[chuckleong21]

addressing #28

[chuckleong21]

Some notable changes/enhancements:

• Save a copy on installed packages strategy has now removed and depends on list_packages();
• admin_password argument removed for reason mentioned in #28;
• file argument removed to reduce redundancy and now updateR() checks whether the file already downloaded in preset download path;
• Preset download for .pkg file is now /Users/whoami/Downloads/;
• Issue #14 is addressed but only for minor releases;
• Compatibility check is added for between the recent R major release and macOS update.