BUG: Stop intraday performance from emitting all transactions.

The intraday performance results were emitting all transactions
for the entire day up to that point, instead of the desired transaction
list for the current timestamp.

Add a `dt` parameter to the `to_dict` method of PerformancePeriod so
that the transactions are limited to a specific datetime.
When the parameter is `None`, a todays_performance object will
function as previously with returning all transactions for the day.
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