Adding option for importing file in one document per line format

One (json) document per line is a common format (e.g. default in `mongoimport`).

Having this import option would ease migration from mongo to rethinkdb.
A potential concern is that `decoder.raw_decode` would not explicitly enforce `json-rows` format; e.g. it would not throw an exception if two json documents are on the same line; or if one of the documents is split between the lines.