Need that for configuring plugins -- originally thought I'd wait until
the class is reworked, but that isn't coming anytime soon given there's
insane pressure on other things, so I have to figure out a plan B.
The interface is the most barebones possible, exposing nothing that I'd
later plan to rename or remove. Just the least possible amount of APIs
to make it possible to configure plugins, and no custom type support yet
either -- if you need to write a vector, write it as a formatted string
instead, sorry.
To make it possible to test the APIs in isolation instead of through
the plugin interface, I added also a Configuration class. Also just the
least amount possible to cover the code with tests.