core/commands/dns: Add 'ipfs dns ...' for resolving DNS references

This lets users resolve (recursively or not) DNS links without pulling
in the other protocols.  That makes an easier, more isolated target
for alternative implemenations, since they don't need to understand
IPNS, proquint, etc. to handle these resolutions.

core/commands/root.go

@@ -41,6 +41,7 @@ ADVANCED COMMANDS
     daemon        Start a long-running daemon process
     mount         Mount an ipfs read-only mountpoint
     name          Publish or resolve IPNS names
+    dns           Resolve DNS links
     pin           Pin objects to local storage
     repo gc       Garbage collect unpinned objects
 
@@ -84,6 +85,7 @@ var rootSubcommands = map[string]*cmds.Command{
 	"config":    ConfigCmd,
 	"dht":       DhtCmd,
 	"diag":      DiagCmd,
+	"dns":       DNSCmd,
 	"get":       GetCmd,
 	"id":        IDCmd,
 	"log":       LogCmd,