Filter with masks/nan

In 1.3 `astropy.convolution.convolve` and `astropy.convolution.convolvefft` will support `NDData` and thus probably also CCDData. We might as well include these somewhere in the narrative documentation, right?

Apart from that I recently "played" around with `numba` and `jinja2` and created a [smallish package](https://github.com/MSeifert04/numbamisc) that should in theory also support `CCDData` and offers sum/average/median/weightedmedian/min/max filters for arrays with masks which can also exclude `NaN`s.

`numba` is definitly not a lightweight dependency so it's not something that should be included in `ccdproc` but would it be a worthwhile addition for the documentation?