The forecast period begins on:
- the start date supplied to the `--forecast` argument, if present
- otherwise, the later of
- the report start date if specified with -b/-p/date:
- the day after the latest normal (non-periodic) transaction in the journal, if any
- otherwise today.
It ends on:
- the end date supplied to the `--forecast` argument, if present
- otherwise the report end date if specified with -e/-p/date:
- otherwise 180 days (6 months) from today.
Note that the previous behaviour did not quite match the documentation,
so this also acts as a bug fix for #1665.
