JPG support with quality option

[l0co]

Why have you removed JPG support? I use jimp composer and in 0.10.2 I can still render image to JPG just by using filename with .jpg extension, but in master I can see there's forced PNG support only (even for .jpg extension a PNG-format file is rendered).

It's useful to have an option to choose the format. For example in my project the example sprite file sizes are:

1. PNG = 4.7 MB
2. JPG = 2.6 MB (quality: 100)
3. JPG = 0.9 MB (quality: 90)

There's no visible quality decrease for these pictures (source images are anyway JPG-s with 90 quality) so I'd like to choose 3rd option with the smallest file.

Also, for 0.10.2 there's no option to provide JPG quality, which I've provided additionally here l0co@7fcd13f