set the min_value of SolidGauge

Thanks for this package,
As i checked this is not any parameter to set the minimum value of SolidGauge. In, my case most of the values are near a big max, so the representation can't pretty show the important differences.

Start by locating the SolidGauge implementation and its existing tests in pygal. Trace how the gauge determines its lower bound, then add support for configuring a minimum value and update tests to cover values clustered near a large maximum.

Written by the indexing model from the issue text.