Media file tags can be customized with the ``write`` event
zero: Only changes media file tags not database

Uses the new API from the previous commit and fixes #963.

There is a possible issue with backwards compatibility: Changes to the
item in the 'write' event do not propagate to the tags anymore. But I'm
not aware of other plugins that use the API in that way.
Merge pull request #965 from geigerzaehler/write-hook-mutate

Zero plugin can modify tags without changing the item