mu-move: add new move sub command

Add sub-command to move messages; add tests and docs.

Fixes #157

mu/mu-cmd.hh

@@ -127,6 +127,15 @@ Result<void> mu_cmd_init(const Options& opts);
  */
 Result<void> mu_cmd_mkdir(const Options& opts);
 
+/**
+ * execute the 'move' command
+ *
+ * @param opts configuration options
+ *
+ * @return Ok() or some error
+ */
+Result<void> mu_cmd_move(Store& store, const Options& opts);
+
 /**
  * execute the 'remove' command
  *