Support CORS headers

[MarhiievHE]

At this moment, API on impress has fixed headers for CORS. This is not always appropriate especially if we want to restrict requests from other websites.
Let's put in the server config the ability to set the Access-Control-Allow-Origin header. For example, adding to the server config:

cors: {
   origin: 'string',
},

Then in the metacom we can use the values from cors.origin in the response headers.