Redesign the dataloader

[oeway]

Currently, we use json schema to match data types, but suffering from performance loss during type checking like we try to fix here: https://github.com/oeway/ImJoy/pull/297 .

We need to revisit the design and try to figure out a more explicit way to doing type checking.

One potential solution is to deprecate the inputs and outputs in the block, and allow users to explicitly register data-loader functions to handle files or data produced by other plugins.

[oeway]

However, allowing custom data-loader would mean all the data will transfer to each plugin which provides the data-loader, which perhaps is not a good idea. We may need to either stick with the schema, or somehow have a safe way to execute the script.