Add --error-if-unmodified option

[Florian Schmaus]

As the motiviation in the commit message states, we would like to be able to 
detect sed invokations that became no-ops. So this is my first attempt in 
adding support to signal this case with a dedicated exit status.

Since this is my first time working with sed's codebase, I may have
missed some cases where the input is mutated. Hence I am looking
forward to your feedback and would be happy if such a functionaly
could be considered for inclusion. :)

- Flow