Support the missing `update` command for major swagger spec file changes

[subeeshcbabu-zz]

Add the update command to support following:

1. add new operation to an existing path
2. add new response to an existing path.

[scorpiodawg]

Hi @subeeshcbabu any ETA for this? It would be pretty useful to have. My workflow has been:

1. design APIs
2. translate a few to Swagger
3. test it out by running the server
4. make some changes to ensure this model works for me before committing to the tools
5. fill in the rest of the Swagger spec, and
6. regenerate the missing pieces

I'm stuck in #6 now, and found this issue.

Thanks for all the good work.