code: public PreferredValueMakersRegistry add(Matcher<?> settableMatcher, Maker<?> maker) {
          Preconditions.checkNotNull(settableMatcher);
          Preconditions.checkNotNull(maker);
          makers.put(settableMatcher, maker);
          return this;
      }
docs: Add a maker with custom matcher.
  
  @param settableMatcher
  a matcher to match on com.github.huangp.entityunit.util.Settable#fullyQualifiedName()
  @param maker
  custom maker
  @return this