NIDM results extractor and demo

[mih]

• [ ] be able to point https://github.com/incf-nidash/nidmresults-fsl to the root of an analysis result and build result NIDM packs. Start here: https://github.com/incf-nidash/nidmresults-fsl/blob/master/bin/nidmfsl#L42-L46
• [ ] open the packs and extract JSON-LD (see here https://github.com/datalad/datalad-neuroimaging/pull/41/files but now nidm.json)
• [ ] ping @cmaumet at this point
• [ ] yield that as metadata, but decide whether it is for a dataset or for the result pack
• [ ] possibly associate parts of the metadata with individual files (e.g. zmaps) -- need to see the actual structure to decide
• [ ] implement a demo query

Metadata structure overview: http://nidm.nidash.org/specs/nidm-results_130.html#fig-nidmresults-core-uml