Read FITS image mode and size

[radarhere]

Helps #4054 in a minor way.

There is currently a FIXME in FitsStubImagePlugin

Pillow/src/PIL/FitsStubImagePlugin.py

Lines 47 to 48 in 45003b7

	# FIXME: add more sanity checks here; mandatory header items
	# include SIMPLE, BITPIX, NAXIS, etc.

Using the following links as references, this PR checks those header items, getting the mode and size.

http://www.astro.sunysb.edu/fwalter/AST443/fits.html

The header is a sequence of 80 character ASCII card images. Each line of the header consists of a keyword and a value; the header ends with the keyword END. A FITS header must contain certain keywords in order for the data to be interpretable. These are:

SIMPLE: =T is a simple FITS file, F otherwise
BITPIX: number of bits per word in the data
NAXIS: number of axes in the array. 0 indicates no data follows. If NAXIS > 0, there must follow a series of keywords NAXISn. where n is between 1 and NAXIS, inclusive, giving the length of each axis of the array.

https://fits.gsfc.nasa.gov/fits_primer.html

The keyword names may be up to 8 characters long... The keyword name is (usually) followed by an equals sign
...
SIMPLE = T / file conforms to FITS standard