Allow array configuration values to be extended

[MarcelVersteeg]

Is your feature request related to a problem? Please describe.
I have several web sites that all have their own configuration file. They all include a general configuration file that also sets some default values for e.g. SkipFiles. Now, when the same property is present in the web site's specific configuration file, the values from the general configuration file are overwritten.

Describe the solution you'd like
The values from these array properties are appended to the list from the included file.

Describe alternatives you've considered
Can't think of any.

Additional context
N/A