Add command line usage to the client

[teon]

The client now usable only with UI, provide a possibility to use it from command line.
Best would be to add options and arguments, when specified the UI is not opening.
Like:

• --instances - list instances
• --vpns [instance] list all vpns - if [instance] is given - only from instance
• --connect <vpn_name> - connect to a VPN <vpn_name>
• --disconnect [vpn_name] - disconnect from all connected VPNS or just <vpn_name>
• --status - show current status (if connected and to which instance)

[cpprian]

• defguard_cli binary

• --instances

• --vpns

• connect, disconnect and status arguments need to know about active connections with vpns

• cli cannot send in separate thread stats about vpns

[teon]

@cpprian please add link to current branch

[cpprian]

branch #210 @teon