Adjust images after resizing

After resizing images, it is a good idea to sharpen images. This change adds the `adjust_options` dict to the Sigal configuration and is passed to the [Adjust](https://github.com/matthewwithanm/pilkit/blob/master/pilkit/processors/base.py#L19) PILKit processor. I also refactored the `generate_image` function first because it became a little bit unwieldy to add even more parameters. I hope it's okay, to just pass the settings dict.

All tests pass but as it is a quite invasive change, a more thorough review might be a good idea.