Allowing hyphens in filename key values

[tsalo]

https://groups.google.com/forum/#!topic/bids-discussion/pG7TnsZNfyA

This could be a mean expressing multiple values to a single key

Original authors: @bthirion

[tsalo]

This is related to https://github.com/bids-standard/bids-specification/issues/226 and #17.

[yarikoptic]

FWIW, relates to

• 62

  as in DANDI archive and its convention we allow for - in the entity values.