Add support for the x-order extension.

[psyhatter]

Fix for #458. Can be used when you need to set the order of fields in the model:

    NewPet:
      type: object
      required:
        - name
      properties:
        name:
          type: string
          x-order: 1
        tag:
          type: string
          x-order: 2

[funvit]

I am think readme must be updated too...

[stevenh]

While this works, could it be simplified to just an x-ordered e.g.

    NewPet:
      type: object
      required:
        - name
      properties:
        x-ordered: true    
        name:
          type: string
        tag:
          type: string

This then uses the order of the properties, without having to put a flag on every field?

Alternatively a configuration flag might be nice, so you don't have to scatter x-order... throughout specs.

[Pe-te]

Hi, all those referenced commits don't seem to exists anymore? Will this make it into master soon or any way to check the commits out directly? Is there maybe an alternative way to get ordered properties?

[filintod]

I created this PR https://github.com/deepmap/oapi-codegen/pull/1190 based off this one, resolving issues with master and minor refactors

[jamietanna]

Closed by #1190