Refactor tweetset_loader.py for greater flexibility

[dolsysmith]

• Move from argparse to click
• Move less-commonly used options to config
• Add logic for creating extracts without loading

[lwrubel]

Add option to specify concatenating JSON files up to a size.

Also move parse_size() from spark_utils.py to utils.py since it will need to be called to supply to_size value.