initializers should be stoppable, similar to component

[bhauman]

think about the design of :figwheel.main/initializers they are really processes that start when a build starts and end when a build ends

This could be expanded to more of a plugin architecture, where it adds hooks (ie pre-build-hook) via multimethods, The problem of state soon comes in for starting and stopping and we get back to component. I really don't want to bring component into this.

So how about we return a stop fn from the initializer. Will this pattern hold up or will we need to return to something more like component?