add remote storage

[joehand]

Add ability to use remote storage things like https://github.com/datproject/dat-http.

I think there were two parts:

1. Deciding on remote storage
2. Replicating to local storage

[joehand]

I think @maxogden and I talked about an API for this but can't remember...

[TemaSM]

@joehand Some news? We are facing issues with dat caused by ISP firewall blocks (something like DPI), so http storage/transport will probably fix it.