Support more types for default values when unmarshaling

[x-hgg-x]

Is your feature request related to a problem? Please describe.
I would like more supported types for default values, so that the following code works:

type color struct {
	R, G, B uint `default:"128"`
}

tree, _ := toml.Load("")
var c color
err := tree.Unmarshal(&c)
if err != nil {
	fmt.Println("unmarshal error:", err)
}

Now it only produces this error:

unmarshal error: unsupported field type for default option

Describe the solution you'd like
Add more supported types.