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>

linux/shim/process.go

@@ -0,0 +1,19 @@
+package shim
+
+import (
+	"context"
+
+	"github.com/crosbymichael/console"
+)
+
+type process interface {
+	// Pid returns the pid for the process
+	Pid() int
+	// Resize resizes the process console
+	Resize(ws console.WinSize) error
+	// Exited sets the exit status for the process
+	Exited(status int)
+	// Status returns the exit status
+	Status() int
+	Delete(context.Context) error
+}