Please add --flake8-config option

[ngie-eign]

Being able to directly specify a config file on the command line to pass along to the pytest-flake8 plugin would be a boon in terms of usability.

There are a number of options that might be supported in .flake8 which the corresponding --flake8* options don't support. Being able to specify a custom config (without having to go through the boilerplate of tox.ini) would improve usability for the plugin.