Add configuration for when the burndown chart is updated

[cornelius]

The burndown YAML configuration should get another entry which defines at which time the burndown data is updated. This usually should be set to some time closely after the daily, assuming this is the time of the day, when the board is guaranteed to be in an up to date state. The data is updated when trollolo burndown runs after this time and there has been no update on the same day yet.

On the last day of the sprint the chart is updated whenever the tool is run. It always replaces the data from the same day, even if there already is an update from after the daily. This is to make it possible to update the data with final state after review and possible adjustments necessary at the end of the sprint.

On the first day of the sprint the first run is taken and data is never updated from the same day.