Handle const like enum with only one option

[UzairNoman]

Need a way to handle this problem in the generator: json-editor/json-editor#1446

The inspiration repo implements it with optional paramater --constAsEnum Use enums with a single value when declaring constants.:

https://github.com/YousefED/typescript-json-schema

[vladimir-prisada]

One more vote for this. For example, AWS API Gateway models are defined using the JSON schema draft 4 and it would be very useful to have enum instead of constant in the schema

[domoritz]

Happy to review a pull request. I won't be able to make time to implement this feature since I don't need it for my use case.