Add command timeline-tag to search for statuses matching the tag.

Since `search` only return the list of tags matching the search, this is
helpful to actually see the statuses for a particular tag.

cmd/mstdn/main.go

@@ -255,6 +255,17 @@ func makeApp() *cli.App {
 			Usage:  "show timeline direct",
 			Action: cmdTimelineDirect,
 		},
+		{
+			Name:   "timeline-tag",
+			Flags: []cli.Flag{
+				cli.BoolFlag{
+					Name:  "local",
+					Usage: "local tags only",
+				},
+                        },
+			Usage:  "show tagged timeline",
+			Action: cmdTimelineHashtag,
+		},
 		{
 			Name:   "notification",
 			Usage:  "show notification",