title: spacy: add lemmatizer lookup for Dutch (nl) language
body: <p><em>I'am using Spacy 2.0.11 with Dutch language model nl_core_news_sm (nl).  How can I add the lemmatization lookup similar to the implementation for German (de)?</em></p>
  
  <p>I tried the following steps:</p>
  
  <ul>
  <li>add lookup to <strong>init</strong>.py in the language folder (nl)</li>
  <li>add lemmatizer.py in the language folder (nl)   </li>
  </ul>
  
  <p>This resulted in the following error after 'nlp = nl_core_news_sm.load()' or 'from spacy.lang.nl import Dutch':</p>
  
  <p>ModuleNotFoundError: No module named 'spacy.lang.nl.lemmatizer'
  ImportError: [E048] Can't import language nl from spacy.lang</p>