Dynamic seasonality

First of all I would like to thank the developers for this awesome package. Works great and it is very straightforward to use.

Lately, I have been thinking of a possible enhancement. Prophet seasonalities are fixed since model beta parameter does not depend on time. However, weekly seasonality often evolves when training a model over several years.

¿Would it be possible to have some kind of piecewise linear beta (like we have in trend) in order to accommodate evolving seasonalities?