Add background color option to simple image writer

[djhoese]

As mentioned in this thread: https://groups.google.com/forum/#!topic/pytroll/5UuL4jjpm3U

When saving a dataset to a JPEG image (which doesn't support transparency) the default output is a white background. We should provide a keyword argument to the save_dataset of the simple image (PILWriter) writer to do any necessary logic to achieve this. Possible solutions would be to overwrite masked values with the necessary color values. This may have to be added to trollimage.

[djhoese]

I don't think this issue makes sense anymore. There is a fill_value parameter which can be set to 0 for a black background or some other value to replace all invalid pixels with that value. Re-reading the linked thread makes me think this is/was an issue with the enhancement that was being used for that data. If someone really wants this feature feel free to comment and tell us about your use case.