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 think ideally, we would be more lenient and accept both formats without the user explicitly setting an option.  This results in a much better user experience, and the code doesn't have to change much.  @neil-rubens, this makes the change significantly different than what you already have here.  If you want to implement this you're perfectly welcome to, otherwise I could take it over in my own branch.  Thanks for all the effort you've put in so far!