Thank for your proposal.
There is a flag that sounds like what you are looking for:
java -jar plantuml.jar -config /path/to/file.puml
However, there could be right now only one single config file (but file.puml can itself contain several !include lines)
Since we find your syntax proposal more clear, so in future version, we added:
java -jar plantuml.jar -I/path/to/file.puml -I/path/to/folder/*.skinparam
A beta with the -I option is available here
This has not been widely tested, so feedback welcome!