Added options 'innerRadius' and 'ringWidth' to draw rings instead of circles

[Severino]

I needed the possibility to draw hollow ring-like semi-circles:

• Prevent stacked semicircles from overlapping (e.g. when using semi-transparent color)
• Creating gaps between the overlapping circles.

This is accomplished by simply using the two option parameters innerRadius or ringWidth alongside the
startAngle and stopAngle options.

• innerRadius: set's the inner radius from the center of the semicircle
• ringWidth: is an alternative Parameter that allows for setting the ringWidth directly: innerRadius = outerRadius - ringWidth

Preview can be seen here:
https://severino.github.io/Leaflet-semiring/examples/semirings.html

[jayli3n]

Very cool feature, hope owner will merge this in