Add options, fix typo in defaults

* Capitalization mismatch between initial config and the setting that is read causes the path to the flake8 executable to never be picked up

* Added option to specify maximum line length and ignore specified error codes

* Specified a default max line length of 79 chars (PEP8 default) and report all error codes by default
i just edited this line to `linter-flake8`. Ok?