Add code to handle config files specified with the --prepend-config
argument.
Options specified in prepend files get a higher precedence than flake8
defaults but a lower one than user configs. This allows to override
the defaults without affecting the behaviour of user and local config
files.
This implements a solution for issue #349
This relies on two things:
1. Properly configuring flake8-import-order to use that style
2. Properly configuring flake8-import-order to know that flake8 is our
application name.
