Handle multiple transformers on the same request

[brendo]

Closes #37

This handles a bug raised by @kmeinke some time ago.

I don't believe the fix is as difficult as thought:

• $transformer is a private variable. If someone's using it, it's on them 🙅‍♂️
• This value is not persisted to disk for each datasource, so there's no migration required and simply updating the extension will enable them to get the benefit.

[brendo]

Bump too 👍 Similar deal, I think this can be merged.