add support for CORS

[antoinerg]

Adds a --cors option to enable Cross-Origin Resource Sharing (CORS).

This may be out of scope for Orca since it can be handled at the HTTP layer by a downstream server but it was also very simple to add here.

[etpinard]

Very cool. Two comments:

• Would it better to enable CORS by default? Strictly-speaking this would be a breaking change, so the way you have it here is 👍 for v1. I'm interested in hearing your thoughts on this.
• This probably deserves a test. An integration test might be nice, but I think an unit similar to this one would suffice.

[antoinerg]

@etpinard I have never seen CORS enabled by default in any HTTP service since it can be a security risk. I think I would leave it off in case we manage sensitive information in Orca down the road.

[etpinard]

Beautiful.

💃