Add a helper that checks to see if FullStory is already initiated

[ralfstory]

This will help prevent reinitialization and reduce the subsequent noisy logs by being able to check the initialization status and calling FS.init conditionally.

[Iodine-]

I can't find anywhere in the docs where it says this specifically, but I have been using window._fs_initialized to do what you described

[ralfstory]

That definitely works!! Though I think it should be exposed through the SDK explicitly - the fact that you've also had a use case for this means it should probably be baked in!

[luser]

Fixed by #130.