Add an additional {} syntax for defining strings in the configuration

file, making it much tidier to define commands that contain other tmux
or shell commands (like if-shell). Also tweak bind-key to expect a
string if it is only given one argument, so {} can be used with it as
well. From Avi Halachmi.

arguments.c

@@ -207,7 +207,7 @@ args_print(struct args *args)
 char *
 args_escape(const char *s)
 {
-	static const char	 quoted[] = " #\"';$";
+	static const char	 quoted[] = " #\"';${}";
 	char			*escaped, *result;
 	int			 flags;