add auto create wp user option

[asdsteven]

trafficstars

Consider the following situation:

1. A user uses Discourse to login Wordpress
2. Sync with email is enabled
3. However, the user does not exist in Wordpress yet

In the current design, a new user will always be created in Wordpress. This is undesirable in the case when the site admin would like to allow only certain Discourse users to login to Wordpress (as content managers/admin/etc).

Therefore, this pull request adds a checkbox option. The Wordpress admin may decide whether a new user should be created in Wordpress. The Wordpress admin may manually create Wordpress users for the other content managers/admins who may login using Discourse.