Takes product ids (store-prefixed, as returned in product results) or a
query; with a query, the closest matching products are looked up first. A
range returns one row per product per day, and each row costs one credit;
with from_date equal to to_date, one day's price is returned.

Args:
    product_ids: e.g. ["tesco:300793546", "dunnes:100873410"]; each id's
        store is taken from the id.
    query: product name words, used when no product_ids are given.
    store: aldi, tesco, supervalu or dunnes, to narrow a query. Leave it
        out to look in all four.
    from_date: first day, YYYY-MM-DD. Defaults to 30 days before to_date.
    to_date: last day, YYYY-MM-DD. Defaults to today.
    limit: most rows to return in total (default 60).

Returns one row per product per day: id, name, store, scrape_date, price,
unit_price, unit_basis, clubcard_price, offer_text, was_price.