support multiple remotes

[sbrudenell]

we should support storing backups on multiple different clouds, to mitigate platform related risks.

looking at various object storage platforms right now, it looks like they all have free ingress and all (except b2) have costly egress. so I expect the naive strategy of just copying from the host to all configured remotes will be actually be pretty good.

this will probably be an improvement on cloud-to-cloud systems like AWS datasync, because we'll be able to e.g. customize storage classes per cloud.

[sbrudenell]

I should note that some very reasonable choices for pipe_through, like gpg encrypt, produce unstable output.

it would be unfortunate to always produce different output on different remotes. it would be nice to tee the output. this is a little easier to do with #32