feat: support for negative values / pan-knob style

[suterma]

Currently the knob does not support negative values. This could be very useful however for e.g. pan knobs (left/right channel distributions), parametric equalizer knobs.

This could be partly solved with the other feature of text templates, rendering negative values for part of the range. However, the highlighted area (rim) for these situations should extend from the center, not the lower left corner.

There is already a knob supporting this feature (https://www.npmjs.com/package/vue-knob-control), but it's of a different look and feel and only supports vue2 at the moment.

[Uninen]

This is a very good point, indeed a needed feature. I think we should support arbitrary values, including negative ones 👍

(This isn't supported yet just because I didn't need it at the time I decided to write this!)