Local configuration files are ignored if `parse_argv` is False

When using `pep8` as a library, there is currently no way to make the `StyleGuide` read local configuration files (`tox.ini`, `setup.cfg`) other than passing `parse_argv=True`. This makes little sense in library use where `sys.argv` is empty, since it [breaks for the case where no local configuration files exist](https://github.com/jcrocholl/pep8/blob/master/pep8.py#L1840) with "input not specified".