Would be cool if you add support OpenAPI

[sah4ez]

When I tried generate change log I got an error:

(node:14754) UnhandledPromiseRejectionWarning: SyntaxError: /home/sah4ez/go/src/bitbucket.org/ronte/tmp/swagger.yaml is not a valid Swagger API definition
    at /home/sah4ez/.nvm/versions/node/v10.0.0/lib/node_modules/swagger-changelog/node_modules/swagger-parser/lib/index.js:75:19
(node:14754) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 4)
(node:14754) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.