code: protected void irregular(String singular, String plural) {
          plural = lower(plural);
          singular = lower(singular);
          plural(newIrregularRule(singular, plural, locale));
          singular(newIrregularRule(plural, singular, locale));
      }
docs: Defines a word with irregular plural. Ensures that the case of
  the first letter is taken over.
  <pre> {@code
  irregular("cow", "kine");
  pluralOf("cow"); // -> "kine"
  pluralOf("Cow"); // -> "Kine"
  singularOf("kine"); // -> "cow"
  singularOf("Kine"); // -> "Cow"
  }</pre>
  The implementation of this method relies on
  {@code plural} and {@code singular}.
  @param singular
  @param plural
  @see #plural(org.cthul.strings.plural.RulePluralizer.Rule)
  @see #singular(org.cthul.strings.plural.RulePluralizer.Rule)