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.
> My understanding has been that output processing should be strict, not input.

Its a difficult call; what do you think @Tryneus , @coffeemug ?

>  a decoder.raw_decoder based loop could support all 3 formats accepted in a single function.

There is probably a chance to re-factor things a little; e.g. calling `read_json_single_object` from `read_json_array`.  

To handle `json-rows` I made a new function, as not to introduce any side-effects; since there  additional i/o handling done in `read_json_array` (for buffering, scanning, etc.).