writep8 arg producing png not p8
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'
```