[optionally] add "vendor" prefix to custom keywords

[epoberezkin]

trafficstars

Possible approaches:

• simply rename all custom keywords, e.g. "switch" to "x-switch" or to "ajv-switch"
• add the ability to redefine names (or to define prefix) at the time keywords are added

[iainsproat]

Json Schema's preferred method is to add an x- prefix: https://json-schema.org/blog/posts/custom-annotations-will-continue