--rebuild arg to export scripts should rebuild parents

[jeancochrane]

We discovered today that when running scripts/export_models.py or scripts/export_qc_town_close_reports.py with the --rebuild argument, the script won't rebuild parents of the models in question, so if you don't have the parents built dbt will raise an error. We should be able to fix this by appending a plus sign (+) to the tag: expressions in the select statement.