Adds a watch flag to the get command

The new flag fetch and display the request ressource and then continue
watching the ressource until timeout or cancellation.

A single ressource/ressource type is supported.

Signed-off-by: Soule BA <soule@weave.works>

cmd/flux/get_source.go

@@ -25,6 +25,10 @@ var getSourceCmd = &cobra.Command{
 	Aliases: []string{"source"},
 	Short:   "Get source statuses",
 	Long:    "The get source sub-commands print the statuses of the sources.",
+	RunE: func(cmd *cobra.Command, args []string) error {
+
+		return validateWatchOption(cmd, "sources")
+	},
 }
 
 func init() {