Remove entire subscription model

It was a valiant effort, and the implementation was solid and
dependable, but at the end of the day we can achieve the same
functionality in a much simpler fashion with commands, especially
because Go is not held to the same restrictions as Elm.

examples/http/main.go

@@ -23,7 +23,7 @@ type statusMsg int
 type errMsg error
 
 func main() {
-	p := boba.NewProgram(initialize, update, view, nil)
+	p := boba.NewProgram(initialize, update, view)
 	if err := p.Start(); err != nil {
 		log.Fatal(err)
 	}