Add middle, end and other song hooks to time_callbacks

[podcasthosting]

Why should this feature be implemented?

This allows us to track that the song has been played up to the middle and to the end for analytics.

Feature Description

Instead of integers we could define certain keywords ("first_quartile", "middle", "last_quartile", "end"):

    "time_callbacks": {
        1: function(){
        },
        "middle": function(){
        },
        "end": function(){
        },
    }