Add new GitHub parser called `WikiUpdateEventParser`.

[BURG3R5]

This issue depends upon the issues #2 and #3 and thus can be only attempted after those issues are closed.

In this issue, you'll add a new GitHub parser class called WikiUpdateEventParser in the file bot/github/github_parsers.py. The data sent to us by GitHub for this event can be found here. The event is called "gollum" but it is the wiki update event we're looking for.

Step 1

Create WikiUpdateEventParser class.

For this step, you'll do the following:

1. Create a class called WikiUpdateEventParser that inherits from EventParser and implements the static methods verify_payload and cast_payload_to_event just like the other parsers in the file.
2. In the verify_payload method of this class, the condition should just check that event_type == "gollum". Nothing else is required.
3. In the cast_payload_to_event method, event_type, repo and user should be assigned properly according to the other parsers. Then, wiki_pages should be assigned to a list of WikiPage objects made up from data inside json["pages"]. Confirm the exact details from the documentation link given above.

Step 2

Update list in parse method

For this step, all you need to do is go to the list event_parsers inside the parse method inside the GitHubPayloadParser class, and then add WikiUpdateEventParser at the end of the list.

This issue is a part of a bigger issue (#1). Leave a comment if you have any doubts.