add the possibility to change public/tiddlers/

[bourgeoa]

Actually tiddlers are stored in a subfolder of /public/tiddlers/, main by default
The idea was that tiddlywiki tiddlers were grouped in a single folder for a pod.

In the long run the pod itself would have indicated where it wants to see tiddlers regrouped.
This is the general intend in solid that pod owner should tell the app where to store data and not the app to decide.