Support a --refresh and --step parameters

[stephen-soltesz]

Queries can be expensive. It may be helpful in some cases to run a query that adds a timestamp column. The interval of timestamps would be smaller than the --refresh interval.

For example:

timestamp, site, value
2017-11-10 10:00:00, foo01, 100
2017-11-10 11:00:00, foo01, 100

The query would run every --refresh interval, (say 24hrs), and bqe would iterate through the timestamps at --step intervals of an hour. Arguably the step could be determined automatically...

This feature could help work around behavior we've observed from bqe with small refresh intervals where bigquery apparently stops returning updates around midnight (though that symptom could have other causes, such as time-to-availability of bq streaming inserts).