Add "Diff" function

[Drugoy]

Diff function should be able to run a comparison of two styles. This might be extremely useful not only when receiving styles' updates (the user will know what the changes exactly are), but could also be useful at comparing different styles, which is useful for site administrators creating CSS-stylesheets for their sites.

It would be super-hard to implement, so you might postpone this issue ~forever, but this goal is good as a strategic target to move towards to.

[grssam]

Luckily there are open source diff-ers, but still it is very hard, the UI would be challenging too.