Use hoot streaming reader

[maxgrossman]

Providing hootenanny with [out:json] in an overpass query leads it to use a non-streamable json reader. As a result, the convert command ends up trying to buffer all the data it gets from the overpass query into memory before writing it out to a file. The xml reader can be "stream" the incoming data from overpass, so we want to use it.

This change allows the overpass output type to be chosen when the Hootenanny class is initialized. If no choice, parameter, is provided we default to xml.