add support for multiple "allowed hosts" definitions to improve readability

[lgmu]

For example:

# ABC
allowed hosts = 10.0.0.1,10.0.0.2
# DEF
allowed hosts += 10.0.0.3,10.0.0.4

Currently there is no "+=" Syntax and you can only override it

[sni]

this is possible now, just make sure you add commas where you want them. The config lines will simply be joined as text. See an example here: https://github.com/ConSol-Monitoring/snclient/blob/main/docs/configuration/index.md#appending-values