add a way to include custom models in the convert script without code changes

[ssube]

The convert script can automatically download and convert all of the necessary models, but I don't want to maintain an ever-increasing list of custom models or force people to go through a constant git stash cycle every update.

Provide a way to include a list of custom models, probably from a JSON file, in the conversion process. Something like python3 -m onnx_web.convert --diffusion --custom /data/models/custom.json. The file format would most likely match the map of models in the script already.