Options aren't passed to plugin's properly

It seems that the options like 'interlaced: true' etc. aren't passed to the plugin's in the proper format.
This caused JPG and GIF images not to be converted to be progressive/interlaced and setting optimizationLevel for PNG images didn't have any effect.

I've added a test for JPG images to check if they are progressive but unfortunately it depends on ImageMagick (CLI tools need to be installed on the system). Added it in an external file so it can be easily disabled if ImageMagick isn't available.

I've also updated the test for GIF images because passing the interlaced options keeps it the same size as the original.