Add parameter for custom classes directory to check

[onacit]

Sometimes need to let the animal sniff for other classes directory.

          <execution>
            <id>check-signature-java13</id>
            <phase>process-classes</phase>
            <goals>
              <goal>check</goal>
            </goals>
            <configuration>
              <directory>${project.build.directory}/classes-retrotranslated13</directory> <!-- other classes directory -->
              <signature>
                <groupId>org.codehaus.mojo.signature</groupId>
                <artifactId>java13</artifactId>
                <version>1.0</version>
              </signature>
            </configuration>
          </execution>

[onacit]

Seems duplicate of #21.