Add filtering functionality to bids2table

[ReinderVosDeWael]

This PR adds filtering functionality to bids2table. It can be invoked with lists of values (that are considered an OR), or single values (equivalent to a list of a single value). See example usage below.

import bids2table

bids2table.bids2table(BIDS_DIR, filters={'datatype': ['func', 'anat'], 'suffix': 'T1w'})

Specifically, this PR adds the following:

• A _filter method to _bids2table.py
• Unit tests for _filter, including a workflow to run them
• Updates the example notebook to include the new functionality
• A exceptions.py for custom exceptions.

[clane9]

Superseded by #17.