Update plugin load and snapshot service

Allow plugins to be mapped and returned by their ID.
Add skip plugin to allow plugins to decide whether they should
be loaded.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

services/snapshot/default_unix.go

@@ -0,0 +1,7 @@
+// +build darwin freebsd
+
+package snapshot
+
+const (
+	defaultSnapshotter = "naive"
+)