Add __parse_team_stats to converter.py

This function parses the team stats block and saves the values to the
JSON object.
Add split_on_dashes to parse_team_stats.py

The raw data separates variables in the team stats box with '-', but
then also uses '-' to denote negative numbers. split_on_dashes handles
this correctly, returning a tuple where items proceeded by '--' are
marked as negative. Included is a new test for this function.