Provide connector method for term extraction

* VuFindSearch/Backend/Solr/Connector.php (terms): New method. Extract
  index terms.
* VuFindSearch/Backend/Solr/Backend.php (terms): Use connector method.
Support `identity` munge

* VuFindSearch/Backend/Solr/SearchHandler.php (mungeValues): Support
  `identity` munge.

Identity munge uses the search string as-is.
Document identity munge

* ../config/vufind/searchspecs.yaml: Document identity munge.