TFLite: Support custom attributes

[lgeiger]

Netron is awesome for viewing TFLite models, however the custom ops don't correctly render custom options, but show the raw uint8 flexbuffer array which is not easily human readable:

It would be great if Netron would correctly render custom felxbuffer options. Recently there was an official JS Flexbuffer implementation added that could be used to convert the raw data into human readable key value pairs.

I attached an example network with custom ops that could be used for testing.

[lutzroeder]

Duplicate of #499

[lgeiger]

Sorry I missed #499.

I briefly looked into fixing this issue. For reference: it looks like there is currently a bug in JS flexbuffer support google/flatbuffers#5949 (comment) that prevents this from working correctly with the example network I posted.