Add support for custom comparators

[kotovdv]

It must be possible to create comparison validation logic for classes, that do not implement Comparable interface directly, but instead they do have valid Comparator class.

             ensureThat(Subject.class)
            .field("myField")
            .isGreaterThan(nonComparableField)
            .using(new NonComparableFieldComparator<>())
            .build();