Add an optional "Cancel" button to HPM2's iframe

[rlacorne]

It is a very common pattern nowadays to have multiple step forms, with the possibility to navigate backwards and forwards between steps.

It would be a very useful inclusion to add an optional button before the already implemented submit button, in which the developper could provide custom functions, in the same way we can customize the callback function at the moment.  

This would allow the iframe to fit more naturally within many more web environments and ux flows.