Cross-origin resource sharing

Add feature to persist cors hosts in config, edit in general settings tab and append to API headers if any is set.
@halfdan @czolnowski did you test that it works when several hosts are added to config file? I was reading several SO answers that comma delimited didn't use to work, but maybe it's fixed since. 
Instead solution may be to set the `Access-Control-Allow-Origin:` when needed http://stackoverflow.com/a/7454204/3759928