new feature: return version and exit

[Robert Riemann]

Dear all,

thank you for this tool. make install is entirely silent. So I thought of querying for the version after installation:

snap-sync --version

snap-sync version 0.7, Copyright (C) 2016-2021 Wes Barnett
snap-sync comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it under certain conditions. See the license for more information. 

==> ERROR: Unknown option: '--version'. Run 'snap-sync -h' for valid options.
/usr/bin/snap-sync: line 59: notify_error: command not found
Exited due to error on line 59.
exit status: 127
command: notify_error 'Error' 'Check journal for more information.'
bash line: 63
function name: error

I'd be happy to have a --version option. :)

Best,
Robert