Support converting P mode PNGs

[yzhs]

Figured out the limitation in PIL: P mode PNGs cannot be converted to JPEG. When first converting to RGB(A) mode, it works, though.

Stackoverflow has the answer, as always. :-)

Adds conditional conversion to RGBA mode and removes guards introduced in #30.

Tested locally with this screenshot:

python imgp --convert --overwrite screenshot.png
# and
python imgp --convert --overwrite --rotate 90 screenshot.png

Fixes #50

[jarun]

Thank you!