Added pointer dereferencing for field extraction

[lmika]

Hi @antonmedv,

Saw this on Golang Weekly and though it looked pretty good. Added a PR which dereferences pointers to structs when attempting to extract a field name so that pointers to structs can be used as data alongside regular structs. Example:

type Data struct {
   X int
}

data := &Data{X: 123}
result, err := expr.Eval("X", data)
// result == 123

Hope you find it useful.

Thanks,

Leon

[antonmedv]

Really cool! Thanks!