Add an agency cli arg

[heijul]

When using --non-interactive, or when it is clear which agency should be used, having to input the agency, once reading is complete is a hassle.
Therefore we should add a cli arg --agency, where the user can supply the agency id to use, or in case a DummyAgency should be created an empty string/-1/something similar.
Need to also raise an error, if the given agency id does not exist in the given agency.txt.