Allow python loader to specify environment variable name

Hi,

As today with the python loader you can only specify in which module to look after your environment-object and the variable is hard coded to be just "environment" [1]. 

With this pull request, I've added the possibility to specify the variable name of the environment by prefix (module:variable_name), like how it works with most wsgi servers (eg. gunicorn module:app).

[1] https://github.com/miracle2k/webassets/blob/master/src/webassets/loaders.py#L258