Support http/https nested requirements

Hi,

Thanks for this lib, it is very useful !

I was trying to use it in `pip-deepfreeze` as a replacement of my own copy of the pip parser (https://github.com/sbidoul/pip-deepfreeze/pull/107), when I realized it only supports nested requirements that are local files. `pip` does support http and https URLs as nested requirements files.

Would you consider this in scope?

It could be implemented with an optional `fetch` function to be provided by the user and called when nested requirements are `http` or `https`. This way, this project would not need to depend on an http client library nor worry about all the subtleties involved with URL fetching such as authentication, etc.