Move init to 'config init', add config edit/get/set subcommands

The config command group manages the config file:
  config init  - write default config (moved from top-level init)
  config edit  - open the config in $EDITOR (falls back to vi)
  config get   - print a value by dotted YAML path (s3.bucket)
  config set   - set a scalar value by dotted YAML path

get/set operate on the yaml.Node tree so comments and formatting in
the config file are preserved across edits. set creates intermediate
maps as needed.

internal/cli/root.go

@@ -41,7 +41,7 @@ on the source system.`,
 
 	// Add subcommands
 	cmd.AddCommand(
-		NewInitCommand(),
+		NewConfigCommand(),
 		NewRestoreCommand(),
 		NewPruneCommand(),
 		NewStoreCommand(),
@@ -78,7 +78,7 @@ func ResolveConfigPath() (string, error) {
 		}
 	}
 
-	return "", fmt.Errorf("no config file found; run 'vaultik init' to create one, or specify with --config")
+	return "", fmt.Errorf("no config file found; run 'vaultik config init' to create one, or specify with --config")
 }
 
 // defaultConfigPaths returns the ordered list of config paths to search.