Allow downstream programs to supply additional rules

[effigies]

PyBIDS uses the bids_validator Python library to filter files by default. It is not currently possible to augment this with additional rules to allow certain (pre-standard) new files without disabling the filter entirely.

Given that the current API is an object that each caller is likely to instantiate independently, some kind of configuration that would permit each new construction to incorporate the additional rules would be useful.

For example:

>>> import bids_validator
>>> validator = bids_validator.BIDSValidator()
>>> validator.is_bids('/models/model-001_smdl.json')
False

>>> bids_validator.add_rules('statsmodels', 'path/to/statsmodels.json')
>>> 
>>> validator = bids_validator.BIDSValidator()
>>> validator.is_bids('/models/model-001_smdl.json')
True

cc @tyarkoni

[effigies]

Related to poldracklab/fitlins#206.

[sappelhoff]

I guess this is related to https://github.com/bids-standard/bids-validator/issues/826 as well