8.x-0.1: add hook_jsonld_alter_field_mappings (#31)

* add hook_jsonld_alter_field_mappings

* coding standards

* implement hook_jsonld_alter_field_mappings

* cache field mappings and issue warnings for duplicates

* trying to make phpcpd happy

* revised field mappings hook name

* revised field mappings hook name (again)

* update const to match