Specifying prefix / suffix for flow type

[mattgstevens]

I was very pleased to find this library as it does a nice job of generating flow types from an OpenApi schema.

One thing that is missing for me is being able to change the name of the flow type to include a prefix or suffix (different projects I have worked on use one or the other).

ie. changing a schema definition named User:

• with a type prefix like TUser
• with a type suffix like UserT

I think this would be covered by adding two additional arguments to the program to take a string and modify the name in all definitions & "$ref" positions.

Are you be open to a PR that addresses this?

[cloudle]

Created pull request, hope it will be merge/release on npm soon.