Support for custom CORS file

[gipsh]

Background

The CORS config is hardcoded on resources. For a project i need to use a different CORS config.
Same problem mentioned on issue #88

Proposed changes

• support for custom json cors file (like policy.json but for cors)

Proposed reviewers (optional)

[@ mentions of other contributors]

[fernando-mc]

Hi @gipsh! Thanks for the PR, looks like a good one - can you potentially add some documentation on how this works? I can't merge it in without something in the readme explaining how it works. That also helps me test it.

[Shereef]

@gipsh Are you able to add some documentation and maybe some tests ? if you will work on the tests please wait for #144 to be merged first

Please also rebase off of master

Thanks so much

[mikejpeters]

Thank you for the pull request, it was helpful for #160 which includes your changes in addition to updating the docs and tests