Validation of painFormat in the SepaDocument constructor

[lippertto]

I have updated sepa.js to 2.0.0, and I ran into a problem because the pain format that I used was no longer supported.
The root node of the generated xml document was called undefined which is obviously not correct.

I would create a PR to check if the provided painFormat is supported in the constructor and throw an exception if it's not.
Do you think this makes sense?

[kewisch]

Makes sense to me! Note I just committed to accepting a typescript rewrite in #70, though this was just moments ago. We should make sure it survives the rewrite.