Add nice/priority options.

[yichen88]

Signed-off-by: yichen88 tang.yichenyves@gmail.com

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What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
feature

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Add priorty/nice

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[sylvlecl]

@yichen88 : while we are on this topic, maybe we could add the "min-time" option ?