Support `NE-OPTIONAL` tag

From https://github.com/ku-nlp/WikipediaAnnotatedCorpus/blob/main/knp/wiki0025/wiki00252305.knp
```
<NE:OPTIONAL:新幹線総局><NE-OPTIONAL:ORGANIZATION,NONE:0><rel type="ニカンシテ" target="新幹線" sid="wiki00252305-01-01" id="21"/><rel type="ノ" target="鉄道" sid="wiki00252305-01-01" id="9"/>
```