ADD: default_version flag for dataset indexing

Fixes #19

Adds possibility to set default dataset version different than the currently indexed dataset.

Example usage:

python manage.py index --dataset_key=lisn --dataset_version=2.0.0 --dump_dir=dumps --default_version=2.0.1

If version 2.0.1 is already indexed, it is set as the default version to view. If it does not exist, it is ignored and default version is set as 2.0.0.