Add start date of the stats

[sylhare]

As of now it gives the stats for the two previous week, but it would be nice to have the exact start date and end date to avoid confusion.

Plus it could be useful if we decide to give custom date range.

The start date would be the date of the first pipeline in the time range, according to #3 if there are more than a 100 pipelines the 100+ are not counted, so the start date shall be the date of the 100th pipeline.