Lazy publishableKey config

[josemarluedke]

Fixes #53.

[lindyhopchris]

Thanks for this! Looks good.

The only thing that occurs to me is do we know what happens if load is called a second time with a different key? Just wondering what happens as that might be the implication of the original post in #39

[josemarluedke]

That's a good question. I don't know the consequences of that. However, we do prevent calling configure if it was already configured, so I guess that's safe.

I guess this PR does not fix #39.

[lindyhopchris]

Yeah, need to clarify whether toggling the publishable key is a requirement and/or even possible. But this PR is great at the mo so will merge for now.