add function to use custom debian scripts

[danielbisar]

Related issues

https://github.com/astraw/stdeb/issues/132 https://github.com/astraw/stdeb/issues/46

I tried to keep it simple. So I just check in case of bdist_deb if the debian folder exists in the repo (next to the python module folder) and it copies all files present there before executing dpkg-buildpackage. Basically the idea is based on the workaround mentioned in https://github.com/astraw/stdeb/issues/46

There is another PR which adds these scripts as options, but this cannot be merged and is from 2012 https://github.com/astraw/stdeb/pull/50