We are not using the manager yet though.

Allow several modules within a main module.

We also start parsing the output main module (which does not have an
"http" module yet).

We also get the filter function constructor.

We introduce a registry that contains maps to builder functions. These
builder functions return the interface implementation of the modules
specified in the configuration.

We also make the input module type use the registry. All other module
types still have to be converted.

For now it's mostly key-value pairs but it will most likely have to get
more complex because of the if-statements.