Add DefineNumberFormat function

[bgoesswe]

Hey,
We use a custom format with type "number" e.g.:
"eo:epsg": {
"type": "number",
"format": "epsg-code",
...
}

There is the function DefineStringFormat implemented into openapi3 (related issues #103, #79), but it only handles elements with type "string". So there is a need for a DefineNumberFormat function.

[fenollp]

Pars welcomed!