Allow customization of arc thickness and format value label

[askjervold]

When using this extension, we found that the arc could be a bit too thin when the chart was scaled down, especially with the markers thinning it out even more. This scales down the arrow length and font sizes when you supply a different cutoutPercentage so that the arrow doesn't draw over the value and the marker labels are within bounds of the canvas.

Additionally, we found that when we formatted the marker labels, the value label lacking formatting looked very out of place. This also formats the value label using the same function.

[kluverua]

Is this modification compatible with old configs?

[kevinjpetersen]

When will this be merged?