Save and load auto-tuner settings

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Maybe save each tuner setting in a file in $HOME/.yask/ dir or something like that.
Would need to match platform and stencil settings, perhaps using some sort of key.
Should existing setting be replaced automatically or only on demand?
Probably harder than it sounds to make it usable, flexible, and intuitive!

The per-stage tuning feature has been removed--should make this a bit easier to implement.