[Season] Add Seasonal formatter

The formatter is now in reasonable shape.
The season header is not completely ready, and should be aligned with Exceptions header (or vice versa).

Question: Should the 'Season header' yes or no show the start and end dates. I think not. It could be mentioned in the Season name.
We still need to implement recurring seasons, and then, no dates are requested.

This should be OK now. Waiting for some weeks before releasing it, knowing that there is still enough to polish.