allow to set date in the export

[individual-it]

this adds one more flag to the export command, with -d or --date a specific date can be set
That string can be accessed in the template with ExportDate
The flag is optional and will default to now

@phil-davis to use it in our current bill template you would have to set this line
### Detmold {{ now | date "02.01.2006" }}
to
### Detmold {{ mustToDate "2006-01-02" ExportDate | date "02.01.2006" }}