Add support for ffextensions

[chrisbygrave]

This PR adds support for a gostruct tag ffextensions that allows a comma-separated list of key/value pairs to be added as extensions to the schema generated into the OpenAPI document. Keys must be of the format x-<name>.

For example, the following structure:

type TestExtensions struct {
	MyString string `ffstruct:"mystruct" json:"mystring" ffextensions:"x-key=value"`
}

Would generate a schema:

mystring:
  description: mystruct.mystring
  type: string
  x-key: value

[EnriqueL8]

Looks good - a question and needs gofmt formatting

[EnriqueL8]

Looks good to me @chrisbygrave - thanks!