Implemented interlace support for GD, Imagick, and Gmagick.

I thought it best to raise my own PR for Interlace Support given [PR #185](https://github.com/avalanche123/Imagine/pull/185) has been dormant for a month or so.

Apologies @vchrm.

This PR adds interlace support for the GD, Imagick, and Gmagick implementations. It still requires updated documentation and tests, as such, I would really appreciate some guidance on the best approach to add tests for this functionality.

I'm looking forward to having someone review this.

Thanks everyone.

Anthony.