Added new driver virtual methods 'start_driver' and 'stop_driver'

termkey-internal.h

@@ -10,6 +10,8 @@ struct termkey_driver
 {
   void          *(*new_driver)(termkey_t *tk, const char *term);
   void           (*free_driver)(void *);
+  void           (*start_driver)(termkey_t *tk);
+  void           (*stop_driver)(termkey_t *tk);
   termkey_result (*getkey)(termkey_t *tk, termkey_key *key, int force);
 };