Add Reta Vortaro (Esperanto) data source

[Mindful]

We have one Esperanto data source, but Reta Vortaro looks like a much stronger candidate, so we should support it. The repo here has a collection of utilities for processing the Reta Vortaro data, as well as a link to the technical documentation.

[Mindful]

The utilities in the linked repo require Python 2 and don't look feasible to integrate. However, this information is made available via an API at La Simpla Vortaro, which looks easy to use.