[Review]: Added support for gzip compressed FITS files.

Astropy supports reading gzip compressed fits files, so I added those to the _detect_filetype function. This is just checking for extensions, otherwise you would have to decompress the header of the fits file and do the same regex which may be a bit complicated.
If the gzip file check fails, it falls back to previous behaviour.
This is a good point.

@rubendv fancy changing it for correctness' sake?