Add slice and array annotation to struct fields

[artemklevtsov]

This commit adds slice type when used in the struct field.

Should fix #36 and #45.

To reproduce:

package main

import (
	"github.com/kr/pretty"
)

type MyObj struct {
	Name   string
	Fields []MyField
}

type MyField struct {
	Name string
}

func main() {
	o := MyObj{
		Name: "xxx",
		Fields: []MyField{
			{
				Name: "fff",
			},
		},
	}

	pretty.Println(o)
}

Before:

main.MyObj{
    Name:   "xxx",
    Fields: {
        {Name:"fff"},
    },
}

after:

main.MyObj{
    Name:   "xxx",
    Fields: []main.MyField{
        {Name:"fff"},
    },
}