add gdm_add and gdm_remove

[webtax-gh]

added these two functions:

/** Adds a recipient to a Group DM using their access token */
void gdm_add(snowflake channel_id, snowflake user_id, const std::string &access_token, const std::string &nick, command_completion_event_t callback);

/** Removes a recipient from a Group DM */
void gdm_remove(snowflake channel_id, snowflake user_id, command_completion_event_t callback);