Add V for select line with vi(1) keys. From Juho Pohjala.

tmux.h

@@ -712,6 +712,12 @@ LIST_HEAD(joblist, job);
 struct screen_sel {
 	int		 flag;
 	int		 rectflag;
+	enum {
+		LINE_SEL_NONE,
+		LINE_SEL_LEFT_RIGHT,
+		LINE_SEL_RIGHT_LEFT,
+	} lineflag;
+
 	int		 modekeys;
 
 	u_int		 sx;