Added the ability to specify to compare two engines on one url

I've added the ability to compare screenshots of a single URL from two browser engines, currently phantomJS and SlimerJS but it should also work with TrifleJS.

There is also an option to specify a separate snap.js file for each engine to allow pre engine settings to be applied.

I have increased the fuzzy setting within the config file as I found the sensitivity was too high for differences between the browsers renderings but this is obviously still adjustable via the config.

It's a very simple implementation so if there are changes / improvements that should be made / discussed, I'm more than happy for that to be case.

I'm very new to Ruby so any improvements / feedback is much appreciated.