add --apply-template option to ensure desired dock items

Intended to use e.g. on lab PCs as a login-LaunchAgent.
(un)desired dock items can be specified in a JSON 'template' file.
Optionally only affects users explicitly named in template.
Currently invokes dockutil itself (only) when changes are necessary;
this might not be ideal, but delivers expected functionality for now/me.

A colleague asked me about this functionality -- surely it could be implemented 
in a cleaner way -- thought it might be at least worth to share Dawid's idea...

Thank you for dockutil!