add custom_config param to manage environmentfile

[bc-bjoern]

To inject multiple variables to the Environmentfile.

[coveralls]

Coverage increased (+0.5%) to 74.854% when pulling 75bae4b3f13b07bb777c37155421144820a0ace2 on bc-bjoern:environmentfile into d9433d4db166c3fe336e0c3481d04f259b030f29 on biemond:master.