Allow FITS writer to write to an existing `HDUList`?

I was reading the ccdproc docs for the first time in the context of [APE 6](https://github.com/astrofrog/astropy-APEs/pull/1) and hit upon the [section on writing](http://ccdproc.readthedocs.org/en/latest/ccdproc/ccddata.html#getting-data-out) particularly to a FITS file.

The `to_hdu()` method seems potentially useful in its own right, so that's fine. But I wonder if, in addition to writing a CCDData to a new file, the FITS writer could also accept an already open FITS file. That would include options for which HDU to write the data to or to create a new HDU (so basically there are 3 operations: update, insert, or append). I'm not sure what the best default is.

The same idea should pertain to the `Table` writer for FITS as well--just yesterday I had this question come up. So I'm going to suggest something like this over in Astropy too.