Add options for users to override settings on a per-dataset basis

[nthistle]

From @hanslovsky on #1

We should add options/tags for users to override settings on a per data set basis, including the type. The format of that is open for debate, could be json (-D{"dataset":{}}) or a -D<dataset>:key=value[,key=value...] scheme, e.g.

convert -c /n5/container/specification -o /output/n5/specification -Dsome/dataset:type=label,

Once we make a decision what parameters we would like to be able to set, we can choose an appropriately complex scheme.