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.
I don't see the problem with that. It would still require a file with one or more white space separated json documents. My understanding has been that output processing should be strict, not input. Also a `decoder.raw_decoder` based loop could support all 3 formats accepted in a single function.