Add support for interlacing

[bre7]

To support progressive JPEGs.

From the docs:

-interlace
the type of interlacing scheme

Choices are: None, Line, Plane, or Partition. The default is None.
Use Line to create an interlaced PNG or GIF or progressive JPEG image.

I don't think there's a way to determine the current interlacing setting of an image (to allow auto-detection) so this would've to be an extra parameter for ops (or class level maybe).