Datasource for Washington

First pass at implementing the datasource for Washington state.

This requires a url_paths.csv file and a custom fetch class
because some precinct-level results are archived in ZIP files.

The custom fields in url_paths.csv needed to work around the
data quirks are documented in openelex/us/wa/mappings/README.md.

PDF results still need to be converted and there are some cases
where there are multiple precinct-level files and it is unclear
which one to use.

Addresses https://github.com/openelections/core/issues/145