Add support for remapping prefix key

Make it possible to remap the prefix key (default: ctrl-t) by setting
the prefix-ctrl-key variable in the configuration file.

Allowed values are in the range a..z.

Example, to set the prefix key to ctrl-a simply do:

prefix-ctrl-key = a

src/main.c

@@ -95,7 +95,7 @@ int main(int argc, char *argv[])
     tio_printf("tio v%s", VERSION);
     if (interactive_mode)
     {
-        tio_printf("Press ctrl-t q to quit");
+        tio_printf("Press ctrl-%c q to quit", option.prefix_key);
     }
 
     /* Open socket */