Add pane option so Zindex can be set for MiniCharts layer

[bpb824]

No description provided.

[TomasVondrak]

Hello, it would be really awesome, if this issue would be solved. I am currently working on project where it would be handy. I am relatively new to leaflet (and so to contributing to open source github projects), so I am not sure if I understand everything right. But I was hoping that if minicharts inherits from circle markers which inherits pane option, then it could be inherited as pane option by minicharts as well relatively easily. I was trying to go through minicharts code to see where the problem can be, but tbh I am lost in files. Can somebody at least navigate me, in which part of files I should start? Thanks!