Add CWD as an option for OpenAPI links

[joewlambeth]

I've added an additional attempt to load a json or yaml file from a relative path. In addition to the existing relative path, this will also check for the existing of a OpenAPI spec relative to the optional cwd parameter provided. This is useful with the MkDocs OpenAPI plugin. I've raised a PR there to utilize this.

[RobertoPrevato]

Hi @joewlambeth
I like the change you propose here, thank you! I merge and I'll release a new version.