Add option to disable fetching of the index

[foresterre]

We currently always update the index. We should however make this optional (in which case the user bears the responsibility of having an up-to-date index in place at the right location on disk.

NB: the location on disk is currently not configurable, allowing for the location to be changed depends on foresterre/rust-releases#40

[Animeshz]

Does it mean extending the config::ReleaseSource to be an enum type with an additional location field? I guess that will do it.

[foresterre]

Does it mean extending the config::ReleaseSource to be an enum type with an additional location field? I guess that will do it.

That's an option, or we can make it a separate field. Before we can make this change (without unpretty work arounds) I have to add the option to rust-releases which I hope to do next week.