Add Jimp image operations

[j433866]

As requested in #451 , this PR adds some image operations to better make use of the Jimp module.

I've added:

• Blur image (both fast blur and gaussian blur)
• Contain image
• Cover image
• Crop image (with automatic cropping option)
• Dither image
• Flip image
• Image brightness / contrast
• Image filter (greyscale or sepia)
• Image Hue / Saturation / Lightness
• Image opacity
• Invert image
• Resize image (with an option to maintain aspect ratio, so a scale image operation isn't needed)
• Rotate image

I haven't added a print text operation, as I can't work out how to get Jimp's built in bitmap fonts to be available on the client side.

I've also added min, max and step options to the 'number' argument type to make it less likely for someone to input the wrong value.