Add support for method of map.

[szyhf]

Hi, I find out that while calling getFunc(...), when the kind of val is Map, will only assume the method is provide by k-v. But even a map can have it is own method, and it will help a lot.
For example:

type ExprParams map[string]interface{}

func (e ExprParams) Max(a, b float64) float64 {
	return math.Max(a, b)
}

type User struct{
    Age int
}

func demo(){
    expr.Eval("Max(Teacher.Age, Student.Age)",ExprParams{
		"Teacher":User{Age:28},
		"Student":Student{Age:8},
	})
}

In this cases, ExprParams can provide default method ( like Max) easily.

And the implement is very simple, I hope it can improve the lib.

Thanks a lot.

PS: another question
I find that the only to calling for getFunc(val interface{}, i interface{}), the param i is both string, why not just change the getFunc(...) to getFunc(val interface{}, funcName string) ? That will save the cost of reflect.ValueOf(i).String()

[antonmedv]

Awesome! Can you add tests for this case?

[szyhf]

Sure, but where may I put the test case ?