(feat) Impl function missing, #170

[killme2008]

实现 FunctionMissing 扩展接口：

public interface FunctionMissing {
  /**
   * Called when function not found, return the invocation result.
   *
   * @param name function name
   * @param env invocation env
   * @param args invocation arguments.
   * @return The invocation result.
   */
  AviatorObject onFunctionMissing(String name, Map<String, Object> env, AviatorObject... args);
}

当函数未找到的情况下将调用设置的 onFunctionMissing 处理器，用户可以自定义函数查找和调用逻辑。 #170

提供了一个 JavaMethodReflectionFunctionMissing 实现，采用反射方式调用第一个函数参数的实例方法：

 private AviatorEvaluatorInstance instance;

  @Before
  public void setup() {
    this.instance = AviatorEvaluator.newInstance();
    this.instance.setFunctionMissing(JavaMethodReflectionFunctionMissing.getInstance());
  }

那么就可以直接调用一些 java 方法了：

    assertEquals(2, this.instance.execute("indexOf('hello','l')"));
    assertEquals("heeeo", this.instance.execute("replaceAll('hello','l','e')"));
    assertEquals("hello", this.instance.execute("trim(' hello ')"));
    assertEquals(1, this.instance.execute("signum(3M)"));
    assertEquals(101L, this.instance.execute("execute(__instance__, '1+100')"));