Download, parse and cache schema from urls

[rohit-gohri]

Allow passing a url into the spec field. It should automatically download it and treat it just like passing a local file allowing for SSR with it.

We should add this as warning into the docs, that the docs may not live update and will require a rebuild. If someone wants live preview then they should use specUrl option instead

[rohit-gohri]

I have released a beta for v1 where this is fixed: #146

It has some breaking changes (mostly simplification of option names). New docs are at https://redocusaurus-v1.vercel.app/docs

It will be great if you could try it and give some feedback.