Add support for prefixes and suffixes on the center ringchart label
Merge pull request #1 from KOGI/feature/suffixes

Add support for prefixes and suffixes on the center ringchart label
Calculate radius as 1/2 Diameter instead of 1/5. I assume this was originally done for whitespace/padding, but the css should be the one to handle that.
Make ring thickness configurable via HTML data attribute: `data-thickness="x"` (units assumed as pixels, defaults to 20)