Displaying a diff when the assert fails

[aaronpowell]

Summary of the changes (Less than 80 chars)

Generate a diff between the expected/actual snapshot and displaying that on failure.

Addresses #136

[SimonCropp]

note that https://github.com/VerifyTests/DiffEngine is designed to handle opening diffs for many snapshotting libs

[mlstubblefield]

It'd be awesome if this could be part of the library!

[SimonCropp]

@mlstubblefield or you could move to https://github.com/VerifyTests/Verify 😉