Add --save-json option to futhark-autotune.
Add --save-json option to futhark-autotune.

HIPERFIT/futhark,diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,HIPERFIT/futhark,diku-dk/futhark,HIPERFIT/futhark