Helpers for adding or removing keys from a permission

[brandonfancher]

When rekeying a permission on an EOSIO account, an updateauth action must be issued. Newcomers often make the mistake of thinking that an updateauth will add a given key to their permission. They are surprised to find that it actually replaces the entire permission! For example, if you have three keys on the active permission and intend to add a fourth by issuing an updateauth for that permission containing only the new key, the other three keys will be removed!

Usually, developers are trying to add or remove a given key or keys. Any thoughts on adding helpers for these actions to this SDK? Maybe addKeys or removeKeys methods? They would, of course, take parameters as necessary for the target permission, the weight, etc.