Plugins are loaded only if the config file is explicitly given

This is the problem: https://github.com/python/mypy/blob/e4d1dc49091acbf597e60618de5ff4c5443d9c20/mypy/build.py#L353

If I run mypy just using `mypy a.py` with a mypy.ini in the current directory, `options.config_file` will for some reason be `None` and no plugins will be loaded. `options.plugins` will be set properly though.