make cli for running a single slide (not a directory of slides)

[kaczmarj]

this will be used for the qupath integration. it can look like this:

wsinfer runsingle --wsi path/to/slide.svs --roi "{GEOJSON OBJECT}" --model resnet34 --weights TCGA-BRCA-v1 --results resultsdir

related to #118

[kaczmarj]

we no longer have a use for this as qupath inference uses a separate java-based extension.