Support querying across multiple days in historical queries

[kmax12]

Currently, the historical API only supports querying by one day at a time.

Current API:

caiso.get_historical_demand("Jan 1, 2020")

Proposed API:

caiso.get_historical_demand(start="Jan 1, 2020", end="Jan 30, 2020")

Implementation

Some ISOs have APIs where querying for multiple days at once is more efficient. In those cases, that API should be used. Otherwise, this API can be thought of as was a way to avoid multiple method calls.