interlace option for IM thumb driver

[timnarr]

Would be nice to have an interlace option for the IM thumb driver to create progressive jpegs

http://www.imagemagick.org/script/command-line-options.php#interlace

successfully tested it (for global use) with adding this line in thumb.php (line 278)
$command[] = '-interlace line';

[bastianallgeier]

I just implemented it on the develop branch.