Move plugin registration to separate package

Only compile in plugin support on non-windows 1.8.
Re-enable go 1.7.x tests.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

plugin/plugin_go18.go

@@ -0,0 +1,46 @@
+// +build go1.8,!windows
+
+package plugin
+
+import (
+	"fmt"
+	"path/filepath"
+	"plugin"
+	"runtime"
+)
+
+// loadPlugins loads all plugins for the OS and Arch
+// that containerd is built for inside the provided path
+func loadPlugins(path string) error {
+	abs, err := filepath.Abs(path)
+	if err != nil {
+		return err
+	}
+	pattern := filepath.Join(abs, fmt.Sprintf(
+		"*-%s-%s.%s",
+		runtime.GOOS,
+		runtime.GOARCH,
+		getLibExt(),
+	))
+	libs, err := filepath.Glob(pattern)
+	if err != nil {
+		return err
+	}
+	for _, lib := range libs {
+		if _, err := plugin.Open(lib); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// getLibExt returns a platform specific lib extension for
+// the platform that containerd is running on
+func getLibExt() string {
+	switch runtime.GOOS {
+	case "windows":
+		return "dll"
+	default:
+		return "so"
+	}
+}