Custom transformations for mappers

I'd like to add my own custom transformations in certain cases.

For example look at vischous/singer-sdk!1 (diffs)

In this case I have a very specific transformation tied to Active Directory that I want to do. I'd like to be able to add these types of functions easily without having to add them to my own fork of the Singer SDK. This type of transformation is not usable to anyone outside of active directory so I don't think it'd ever make it into the SDK.

(Ignore all changes but mapper.py as there's some changes due to cookiecutter stuff with Windows that's completely unrelated to this)