Added support for GIF comment extension

[radarhere]

Resolves #1895

[coveralls]

Changes Unknown when pulling 8298fcc on radarhere:comment into * on python-pillow:master*.

[ytrezq]

A last thing : how would I be able to use this code to add/write comments ?

[radarhere]

im = Image.open('test.gif')
im.info['comment'] = b"Comment text"
im.save('test.gif')

[ytrezq]

I know this isn’t related to pillow but what is the maximum size of a GIF file ?

[radarhere]

I highly suspect that there is no maximum size of a GIF file. Here is a link to the GIF89a specification - https://www.w3.org/Graphics/GIF/spec-gif89a.txt - a brief search doesn't show anything, and if it's not in the specification, then I wouldn't think that there is any kind of universal limit.