Support both OpenAPI and Swagger formats

By default, the library generates files in "OpenAPI 3.0.0" format. But you can specify "swagger" format if your API docs are in this format.
Example:

API_DOCS_TYPE="swagger"  # Note: "openapi" is default type of API docs
API_DOCS_VERSION="2.0"  # Note: "3.0.0" is default version of API docs