Support any information exposed via `json` files

[dbitouze]

trafficstars

(The current feature request is a superset of #7 and #10.)

It would be nice for sphinx-tippy to support any information exposed via json files.

In particular, this would allow us to provide in our French LaTeX FAQ rich hover tips of the CTAN packages, thanks to https://www.ctan.org/help/json/.