mu: add --reindex option for mu index

I.e. without having to reinit explicitly.

mu/mu-options.hh

@@ -172,6 +172,7 @@ struct Options {
 	struct Index {
 		bool	nocleanup;			/**< don't cleanup del'd mails */
 		bool	lazycheck;			/**< don't check uptodate dirs */
+		bool	reindex;			/**< do a full re-index */
 	} index;