resources: graph: Add partial implementation of a graph resource

This is something I've wanted to do for a while, but for the reasons
mentioned in the comments, I've been unable to complete yet. I figured
I'd at least merge what does exist so far in case someone else would
like to pick this up. It's a bit of a brain hurdle / monster, because
the tricky part is refactoring the core engine so that this fits in
nicely. Perhaps someone will have more time and/or less tunnel vision
than I to either merge something or sketch out some ideas on the path
forwards. I think it's a useful goal because if recursive resources are
possible, it could force the core engine into a more elegant design.

Happy hacking!

yamlgraph/gconfig.go

@@ -58,6 +58,7 @@ type Resources struct {
 	Augeas   []*resources.AugeasRes   `yaml:"augeas"`
 	Exec     []*resources.ExecRes     `yaml:"exec"`
 	File     []*resources.FileRes     `yaml:"file"`
+	Graph    []*resources.GraphRes    `yaml:"graph"`
 	Hostname []*resources.HostnameRes `yaml:"hostname"`
 	KV       []*resources.KVRes       `yaml:"kv"`
 	Msg      []*resources.MsgRes      `yaml:"msg"`