mu: add 'label' command + manpage + tests

Add a label command and document it.

mu/mu-cmd.hh

@@ -116,6 +116,16 @@ Result<void> mu_cmd_info(const Mu::Store& store, const Options& opts);
  */
 Result<void> mu_cmd_init(const Options& opts);
 
+/**
+ * execute the 'label' command
+ *
+ * @param store message store object.
+ * @param opts configuration options
+ *
+ * @return Ok() or some error
+ */
+Result<void> mu_cmd_label(Store& store, const Options& opts);
+
 /**
  * execute the 'mkdir' command
  *