At the moment, if one has a custom respondd module which includes custom
fields, Yanic will simply ignore these fields. Communities which have custom
fields have to maintain patches on Yanic to have them available.
This commit allows to define custom fields in the configuration file, which
will cause Yanic to also save the values of these custom fields in its internal
data structures. Output modules can then decide whether they want to include
these fields. For most cases, this should avoid the need for patches in Yanic.