I would like to add support for more geonames filters, specifically 'searchlang'.

This doesn't seem like a difficult task, and there are two obvious approaches:

1. Include the geonames filters to be added as keys in the set supported_kwargs defined in the GeonamesQuery._build_params method in geonames.py. I took this approach in the pull request below, and all geonames tests still pass.

2. Stop controlling what filters are passed to the geonames search webservice, and handle any errors that may result from making a request with filters that aren't allowed. This has the advantage that any additions to the geonames api (with respect to search filters) are automatically available as geonames search filters in Geocoder.

As a related note, the geonames support page
has an incomplete list of geonames filters which are supported by Geocoder. The page should make clear that only the filters listed are supported.