on_cache_flush event
on_cache_flush event

Added on_cache_flush event as suggested in 
https://github.com/concrete5/concrete5/pull/848

matt9mg/concrete5,matt9mg/concrete5,avdevs/concrete5,TimDix/concrete5,avdevs/concrete5,avdevs/concrete5,TimDix/concrete5,MichaelMaar/concrete5,MichaelMaar/concrete5,matt9mg/concrete5,TimDix/concrete5,MichaelMaar/concrete5