Improve documentation and test suite

Currently the documentation in README is incorrect, meaning that anyone following along may get the wrong button type.

In README `data-type` is shown to always be `form`, but this results in incorrect button rendering.  The test suite does not reflect this either.  Tests have been added which ensure the correct attributes get set on the form, so users are taken to the correct PayPal checkout screen.

Documentation is added for payment recurrence when using a subscription button type.

.editor config also had an error, which has been corrected.