Abstract away deprecated options in config

[javorszky]

Currently the list of deprecated items is hardcoded into the method that checks it here:

Ghost/core/server/config/index.js

Line 322 in 1f5a378

var deprecatedItems = ['updateCheck', 'mail.fromaddress'],

I'd like to move it out of there and into the config.set() method mirroring pretty much any other option options.

Please assign to me. :)