String Comparison using diff

[ko1nksm]

It makes multi-line text even easier to read.

[AntoniMarcinek]

Following #138 (comment) I have these questions:

• is handling on the reporter side to include diffs in some reporters but not others?
• can one use git diff on something that is not a part of a git repository?
• I understand that the difficulty is in providing different options for the diff, but maybe you could go step by step and initially enable just diff and fallback to nothing and then gradually add other diff flavours? As a matter of fact this issue was the first to catch my attention when I started investigating shellspec and seems like something that I would like to get ASAP (i.e. faster than the full solution to Some problems with using shellspec for integration tests #121 which I can already work around pretty well with ctest)