Load runtimes dynamically via go1.8 plugins

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

Add registration for more subsystems via plugins

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

Move content service to separate package

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

container.go

@@ -32,3 +32,7 @@ type State interface {
 	// Pid is the main process id for the container
 	Pid() uint32
 }
+
+type ContainerMonitor interface {
+	Monitor(context.Context, Container) error
+}