Consider allowing headers to be overridden at any point in time

[slurmulon]

Right now headers are global and can only be defined once. However, for certain headers, such as @Key and @Tempo, it would be useful to be able to change their value as a track progresses (i.e. "contextual" headers).

There are certainly many implications to think through here, but it's a nice concept. It might be best to just say that @Key and @Tempo no longer make sense as headers (but that's probably less likely).