writep8 arg producing png not p8

[benbaker76]

Using the writep8 arg will create a png not a p8 file.

./p8tool writep8 9970.p8.png
9970.p8.png -> 9970_fmt.p8.png

It should create 9970_fmt.p8 not 9970_fmt.p8.png.
In tool.py need to change

            if fname.endswith('.p8.png'):
                out_fname = fname[:-len('.p8.png')] + '_fmt.p8.png'

to

            if fname.endswith('.p8.png'):
                out_fname = fname[:-len('.p8.png')] + '_fmt.p8'