Support to skip hook_init()

Issue Summary

Hi,

In many cases hook_init() will not be required in order to run just a JS callback, and in our case hook_init() completely breaks because it calls functions from the menu module. Instead of putting logic into the hook_init() impementation to detect that it's running under js.php, I thought it might be neater to simply specify that hook_init() isn't required in hook_js().

Here's the patch to allow this flag.

Default is still to run it.

Thanks