Use bids-specification files for filename patterns

[tsalo]

This may be premature, but in bids-standard/bids-specification#475 we are attempting to build a machine-readable version of the specification that can be versioned and pulled in for other packages, like pybids and bids-validator. While the PR is in early stages, I wanted to make sure that the pybids devs were aware of the effort, so they could provide input if they wanted.

The goal would be to replace the bids.layout config files with parsers that would rely on config files from the official specification. Perhaps there would also be the ability to inject extensions that are not yet official as well.