Support BQ job children

[stephen-soltesz]

BigQuery supports complex "scripting" queries that include multiple results.

For example:

BEGIN
    SELECT 10 AS value_example1;
    SELECT 100 as value_example2;
END

The bigquery-exporter could support this type of query by finding value_* columns from each result set. This could also support complex steps, e.g. write to a table, run query on derived table.

See also: #27