D3JS json export

[Rafiot]

Hello everyone,

I'm not sure if it is of any interest, but just in case: I'm (ab)using ete to generate the tree representing the load of a webpage, but rely on d3js for the rendering (more details here: https://www.lookyloo.eu/docs/index.html)

I use the following class to generate a json compatible with the d3js tree module: https://github.com/Lookyloo/har2tree/blob/main/har2tree/parser.py#L328

It may be something you want to add in the next release. If you're interested, I can probably make a pull request.