add flake8 configuration (style checker)

currently adjusted in a way it does not throw errors and warnings with
the source as it is now.

issues will get fixed over time and the exclude list shall get shorter.

setup.cfg

@@ -1,2 +1,8 @@
 [pytest]
 python_files = testsuite/*.py
+
+[flake8]
+ignore = E123,E126,E127,E129,E203,E221,E226,E231,E241,E265,E301,E302,E303,E713,F401,F403,W291,W293,W391
+max-line-length = 250
+exclude = versioneer.py,docs/conf.py,borg/_version.py
+max-complexity = 100