Command for token retrieval configuration

[heywoodlh]

Would be nice if the client's configuration file had a parameter that could invoke a command to retrieve the token for Attic. For example, something like this:

default-server = "kube"

[servers.kube]
endpoint = "http://attic"
token_command = "pass attic/some-token"

(Maybe it already does and I don't know)

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(Thanks for such a great tool, attic has seriously sped up my Nix builds)