Add thread stub hook for possibility of custom pre-load injection

[planetjon]

The purpose of this PR is to add a hook to the Disqus thread stub, allowing for customized pre-load behaviours.

Example, adding a loading animation or text before being replaced when Disqus is loaded. This could be useful to clients on a slow connection where even the time to Disqus logo isn't quick. The user may decide to move on if they don't see any hints of a commenting system.