Changes the behavior of initial loading state.

[radfahrer]

When using the checkForExisting option, start the loading state as false and only set loading to true if the script does not already exist.

Resolves #24

[radfahrer]

@hupe1980 I just wanted to reach out to see if you have seen this PR. If you haven't had time to review it, I understand. Let me know if there's anything you need from me.

[hupe1980]

What kind of error do you have exactly?
We actually have to return loading = true until we are sure that the script has been loaded.

[woojunro]

In my opinion, the original code is reasonable since loading state should be true until useEffect checks if the script already exists and ensures the script is loaded.