Implement multi-field mapping generation
- https://www.elastic.co/guide/en/elasticsearch/reference/current/multi-fields.html
- Currently only one mapping to a datatype is respected
- Need strategy of what is primary datatype and what is field type
- Analyzers are already supported as field
(from redmine: issue id 879, created on 2018-01-22tgradl, closed on 2018-08-10)