Add a facility to lemmatize text for search indexing

Greynir makes it easy to lemmatize text. If the parser fails I can fallback to the bintokenizer and get multiple lemmas for all meanings. This makes for a great search index even if there are some extra lemmas there when the parser fails. 

Perhaps Greynir should provide a function out of the box to do this, as it will be a common use case? I can share my code if anyone wants to see it.