ipfs config cmd with git-style get/set

Adds "ipfs config" command designed in git style.
See specific config's values with:

        $ipfs config datastore.path
        ~/.go-ipfs/datastore

Assign a new value with:

        $ipfs config datastore.path ~/.go-ipfs/datastore

Open the config file in your default $EDITOR:

        ipfs config edit

cmd/ipfs/ipfs.go

@@ -41,6 +41,7 @@ Use "ipfs help <command>" for more information about a command.
 		cmdIpfsCat,
 		cmdIpfsLs,
 		cmdIpfsRefs,
+		cmdIpfsConfig,
 		cmdIpfsVersion,
 		cmdIpfsCommands,
 		cmdIpfsMount,