Added Dutch language support (#46)

Author: avanloo01

lang/nl.css

@@ -0,0 +1,4 @@
+:lang(nl) .theorem::before { content: 'Stelling' counter(theorem) '. ' !important; }
+:lang(nl) .lemma::before { content: 'Lemma ' counter(theorem) '. ' !important; }
+:lang(nl) .proof::before { content: 'Bewijs. ' attr(title) !important; }
+:lang(nl) .definition::before { content: 'Defenitie ' counter(definition) '. ' !important; }

languages.html

@@ -48,6 +48,7 @@ <h5>Abstract</h5>
       <option value="it">Italian (it)</option>
       <option value="ja">Japanese (ja)</option>
       <option value="mwl">Mirandese (mwl)</option>
+      <option value="nl">Dutch (nl)</option>
       <option value="pt">Portuguese (pt)</option>
       <option value="ro">Romanian (ro)</option>
       <option value="ru">Russian (ru)</option>