Add config file argument

[aalhour]

Can you please add an argument to supply a Gunicorn config file which specifies more options for Gunicorn?

[jakerobers]

I noticed that most of the gunicorn Application code (https://github.com/benoitc/gunicorn/blob/master/gunicorn/app/base.py#L79) dealt with loading the config file. Instead of copying and pasting, we could extend off of Application instead of BaseApplication.