[feature request] allow load the user customized alert class

[tpeng]

user may develop new Alert class or customize existing one, would be nice to be able load these classes.

[ahrajabi]

I am not pro in this project but I do this with editing graphite-beacon/graphite.py and add custom property. https://github.com/amirhosseinrajabi/graphite-beacon/blob/develop/graphite_beacon/graphite.py This is true way? @klen