Don't register hook callbacks again when site is regenerated

[mojavelinux]

The hook callbacks for pre_render and post_render are getting registered each time the site is regenerated. That's because generators are invoked each time the site is regenerated (even in the case of an incremental build) and we aren't checking whether the hook callbacks were already registered.

If we only register the hook callbacks once, we'll also need to switch to using static methods that delegate to their corresponding methods on the converter instance.

Finally, add a test to verify that the hook callbacks are not registered again when the the site is regenerated.

[mojavelinux]

Resolved by 93fc608 (from PR #120).