Add configurable index to historical requests

[shtrom]

Useful for LeighCurran/AuroraPlusHA#6 and LeighCurran/AuroraPlusHA#9, to retrieve more than the last day.

>>> time.asctime()  # today
'Sun Aug 13 23:07:59 2023'
>>> api.getday()  # same behaviour as before: data for yesterday
>>> api.day['StartDate']
'2023-08-11T14:00:00Z'
>>> api.day['NoDataFlag']
False
>>> api.getday(0)  # we can get the data for today, but it's a bit useless
>>> api.day['StartDate']
'2023-08-12T14:00:00Z'
>>> api.day['NoDataFlag']
True
>>> api.getday(-2)  # we can get data for the day before yesterday
>>> api.day['StartDate']
'2023-08-10T14:00:00Z'
>>> api.day['NoDataFlag']
False
>>> # 
>>> api.getday(2)  # we can request consumption for the day after tomorrow!
>>> api.day['StartDate']
'2023-08-14T14:00:00Z'
>>> api.day['NoDataFlag']
True
```

[shtrom]

@LeighCurran It looks like v1.5.0 is not available in the repo. Would you be able to push it? I'll rebase my changes on that version.