Introduce .flake8 config file for convenient flake8 invoke

[justb4]

flake8 is used within GHC for Python style checking via Travis. flake8 can however be somewhat cumbersome to use as a fully piped command line needs to be provided currently:

find . -name "*.py" | grep -v docs | grep -v migrations | xargs flake8

Also this takes all default flake8 settings. It is more convenient to use a .flake8 config file and later possibly in tox.ini. Now a developer and in Travis only this commandline remains for invoking flake8:

flake8

Also we can more clearly set some less stricter rules like the default 79 char line length.

[justb4]

Done.

[justb4]

Think this is done, closing. Open new issue when needed.