Add option(s) to ignore some check failures, and perhaps add option(s) to be more restrictive

[pflanze]

As mentioned in #8, some checks that currently exit the program with a failure could be lowered to a warning or disabled completely.

OTOH, the program currently does a no-op in some cases (e.g. when a patch file contains 0 diffs--todo verify), where being more strict might help detect problems.

Find all such places, think of a coherent means of handling them, then add the required options.