Allow setting destructor to keys to wget_stringmap_t
- include/wget/wget.h: Add wget_stringmap_key_destructor_t and wget_stringmap_set_key_destructor() declaration.
- libwget/stringmap.c: Add wget_stringmap_set_key_destructor() definition.
Key destructor is there in wget_hashmap_t, so why not in wget_stringmap_t?