Add a callback for getting the full axe-core results object

[gilgold]

Hey!
In some cases, the full axe-core results object is needed (for logging / tracking or other uses)
It would be helpful to have another callback that returns the full results object (similar to how violationsCallback returns the violations object).

I submitted a PR for this - please consider it :)
#68

[sapegin]

I drafted my ideas on the new API: #75. I hope this covers your use case — let me know what you think.