Skip derivatives indexing

[clane9]

bids2table will currently indexes the entire tree under the input root. In particular, this means any derivatives datasets found under root will be indexed. By comparison, PyBIDS provides an option to specify which derivatives should be indexed, or to skip derivative indexing altogether. It might be useful to have some similar functionality.

One option could be to add an exclude arg that takes a glob pattern or list of patterns for directories to skip indexing. These would then get passed through to the Crawler as skip dirs.

[clane9]

Closed by #35.