Custom metadata views per user

[manthey]

PR #1202 added the ability to customize how metadata is presented and added to items. It would be nice if this could be done per user for some metadata (e.g., a rating done by each user). This could be done via a flag in the config file (e.g., perUser: true), where the key would then be <key>-user-<user name>. This might be rendered <title> (<user name>). Another flag would determine if a user sees other user values or not (the config file then provides a natural way to let only certain users see other user values).