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/options.h

@@ -60,6 +60,8 @@ struct option_t
     const char *socket;
     int color;
     bool hex_mode;
+    unsigned char prefix_code;
+    unsigned char prefix_key;
 };
 
 extern struct option_t option;