Add CORS headers to non-preflight OPTIONS too (rs#85)

Non preflight OPTIONS requests should work with CORS too.
This change is removing abort in actual request CORS handler for
OPTIONS request, so the client can receive the necessary headers
on such request.

Use switch instead of if-else chain (rs#66)

Add a note in the README on the breaking change introduced by rs#57

Add go.mod file

Make aligncheck and megacheck happy

Update travis configuration

Implement Access-Control-Allow-Origin *

Use Access-Control-Allow-Origin: * when all origins are allowed and
AllowCredentials is false.

Fix rs#30

Return 200 if Passthrough is disabled (rs#26)

Return OK if passthrough is false