Upload local data in View Online feature

[declann]

I'd like to go to online editor e.g. to debug, whenever I want to see signals, see data after transformations using data viewer.
However as I use local data files this doesn't work smoothly.

It would be nice if local data was embedded into the spec when I hit View Online (maybe with confirmation in case concerned if local data may be sensitive).

Related: #151

[HaveF]

1. if the local data is small, just copy and paste
2. if the local data is large. It would be too hard to embedded. I'm using subset of data (manually) to debug online, especially when I want some interactive features.