add optional "kibana style" delimiter

[ghukill]

Adds optional delimiter when writing as "Kibana style". Where this formerly would default to a comma ,, this would provide a runtime parameter -kd or --kibana-delimiter that would accept a character to use for multivalued cells.

Addresses: https://github.com/taraslayshchuk/es2csv/issues/61

Wholly understand if doesn't fit the library, was a pretty quick hack for our purposes, but thought a PR couldn't hurt. Thanks for a great and useful library.