added postgresql_lang.py

Since I regularly was missing this type of functionality and was always using an ugly workaround, I decided to create a module to add, remove or change Postgresql procedural languages to db's myself.

Tested the module with all options.
This is a replacement of an earlier pull request added to the Ansible repository (pull request add postgresql_lang #8931) and I finally found some time to branch and create a pull request for the new repo.