Add support for merging based on login field

[flowirtz]

As per all-contributors/all-contributors#416 (comment) we should be able to merge based on the login field instead. This should always be there and be unique, while profile may not be unique.

We should add an input parameter that lets the user decide what field should be merged on.
We should also add that to the docs and reference the above discussion to highlight what advantages/disadvantages each approach has.