Define issue types for the nodejs org

[targos]

Now that we have access to the new GitHub issues features (#921), we should decide on the set of issue types that we want for the org.

By default, there are three: Task, Bug and Feature.

We can create a maximum of 25 issue types.
Issue type fields are: name, description, color (selected from a fixed list of 8 colors), and "private repositories only".

The main differences between issue types and labels are:

• An issue can only be assigned to a single type.
• Issue types cannot be customized per-repository.

Owners can see and edit the settings here: https://github.com/organizations/nodejs/settings/issue-types