link to spec to further explain errors and warnings

[perrygeo]

Internally, use the url property to point to the relevant section in the ietf spec

{
  message: "This is an error",
  url: "https://...",
  ...
}

On the command line with default format, perhaps something like

4:  Polygons and MultiPolygons should follow the right-hand rule [a]

0 errors
1 warning

References:
[a] https://...