add support to solve the field type implement Parameters.

[szyhf]

Hi, this library is awesome and really helps a lot.

But while I using I'm wondering to solve situation like some struct has a field of another Parameters, for example:

type DemoParams map[string]interface{}

func (d DemoParams) Get(s string) (interface{}, error) {
	v, ok := d[s]
	if ok {
		return v, nil
	}
	return nil, fmt.Errorf("%s not exist", s)
}

sharedParams := DemoParams{
	"A":  1,
	"B": 2,
	"C": DemoParams{
		"D": 3,
               "E": DemoParams{
		       "F": 4,
               },
	},
}

expr := "A + B + C.D + C.D.E.F"
// got "1+2+3+4"

So I made this PR, I tried to find the test of Parameters but failed, so if any place can I add test please let me know.

Thanks.