Use preset MIN/MAX values

This enables us to do:

$ MAX=10 spark 5 3
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Instead of:

$ spark 5 3
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Which makes comparable sparklines across inputs possible.

https://github.com/markusfisch/spark/commit/61fa633c3223dca56d3ea96b25b42a5ba3d6f330
(cherry picked from commit 61fa633c3223dca56d3ea96b25b42a5ba3d6f330)

Conflicts:

	spark