support Rocket Chat import format

[JensLehmann]

It would be good to support the Rocket Chat import format as described in https://docs.rocket.chat/guides/administration/admin-panel/import/csv . In particular for private channels, the format should be sender user / receiver user / timestamp / message. Moving from the current format to this format is not that easy (because of multiline messages and needing to insert the receiving user as only the sending user is in the current format).

Looking at https://github.com/patrickfav/rocketchat-exporter/blob/master/src/main/java/at/favre/tools/rocketexporter/converter/SlackCsvFormat.java I'm also unsure whether there is a way to implement this for direct messages, which are treated differently than channels. Probably, the ExportFormat interface needs to be extended by another method for direct messages.