Add support for negative progress/value

Would be great to have a `middle/zero point` added, so the gauge can sway into negative zone

Basically same as was implemented here: https://github.com/gonuit/gauge_indicator/pull/16

Start by reviewing the package's current gauge progress/value handling and the referenced gauge_indicator pull request. Clarify how a middle/zero point should be configured and how negative values should render; done means the gauge can visibly enter the negative zone while preserving existing positive behavior.

Written by the indexing model from the issue text.