Make tabby collections configurable

[mih]

#84 bring a metadata extractor that, presently, can also report additional documents (in tabby format) stored in a dataset.#86
Due to the mechanics of tabby, we somehow need to identify the root sheets of each record. In #84 this is hardcoded to dataset.

This seems needlessly restrictive.

We could make this arbitrary, and also open it up to any number of locations. To achieve that, we could have a config variable (say datalad.tabby.collections) that would take any number of relpaths pointing to the root of any collection of tabby records. A companion variable that includes the collection name would identify the root sheet (e.g. datalad.tabby.collection.<name>.root.