Support allOf to split paths into multi files

[amazing-gao]

allOf is support for swagger-ui 3.x to split paths into multi files. I think this is a very import function for us.

paths:
    allOf:
        - $ref: /api/users.yaml
        - $ref: /api/items.yaml

https://github.com/OAI/OpenAPI-Specification/blob/master/guidelines/REUSE.md
swagger-api/swagger-ui#1098
swagger-api/swagger-editor#233

[amazing-gao]

allOf is a invalid open api schema.