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)