Add (re-implement) "export document(s)" feature

[mocobeta]

I have dropped this feature when porting Thinlet UI to JavaFX (later, replaced by Swing.)

Thinlet Lule has "Tools > Export" menu. This wiki describes the feature.
https://github.com/DmitryKey/luke/wiki/Exporting-index-to-xml

I would like to re-think this feature (but cannot take enough time for this now.)
When (re)implementing this feature, file format selection (e.g. export to xml, json or csv/tsv) would be desirable.

[mocobeta]

For contributors: Now Luke is a part of Apache Lucene. Please create an issue (if there is not exist) to Lucene Jira if you want to help with this.