Allow a mapping between labels and ontology term IDs in the header of GFF

[cmungall]

Currently GFF3 does not use SO IDs. Instead features are type by their label. This imposes strictures on SO - SO can't change labels without potentially breaking GFF3 usage.

Inspired by JSON-LD contexts we could have in the header declarations of mappings between values in the type column and SO term IDs. E.g:

# context:
#   transcript: SO:0000673
#   exon: ...
#   ...

Note that this would need to be widely implemented before SO would be able to change labels, but this would be a step in the right direction