Showing reference and current images along with the diff result

[Muritavo]

This PR enables a feature requested by #144 as it's useful for when comparing complex UI changes.

What does it change:

• Added a new property called includeReferenceOnDiff (default: false)
• When the property is enabled, the reference image and the current images are concatenated side by side along with the diff result.

Currently is set as draft because it needs the following adjustments:

Make the property work on the other two diff engines, if someone want to do it, fell free to contribute to this PR as well

[Muritavo]

Adding here so it is documented:

For the GraphicsMagick library there is a method montage, that combines multiple images side by side.

As for the looks-same library, the createDiff without diff property can be used to manually write the file to disk, with the other references