vendor option prefix

[jodygarnett]

Merged as 6179bc9430a221b6bda165061fe81a8b7c85a7fb

[jdeolive]

-1 on the option prefix. This make it look like a yaml list which is confusing.

[tbarsballe]

This project has been donated to geotools community:

• https://github.com/geotools/geotools/tree/master/modules/unsupported/ysld

It will be marked supported when it meets documentation and quality assurance requirements.