Support unmarshal into toml.Tree

[AllenX2018]

Issue: #333

This pr add support of *toml.Tree marshaling/unmarshaling. With this, you can unmarshal parts of a TOML file into an *toml.Tree for later inspection and unmarshaling.

[A-UNDERSCORE-D]

Tested this with some more complex and nested elements (Which probably was covered by your test, but I wanted to try anyway) and it worked perfectly:

package main

import (
	"fmt"
	"go-toml"
)

type test struct {
	A string
	B int
	C float64
	D innerComplex
}

type innerComplex struct {
	Type    string
	Content *toml.Tree
}

type innerSimple struct {
	Stuff    string
	Things   float64
	someMap  map[string]int
	someList []string
}

func main() {
	const testStr = `
	A = "this is A"
	B = 10
	C = 3.14159
	[D]
	Type = "Something"
	[D.content]
	stuff = "thing"
	things = 3.14235
	someList = ["this", "is", "a", "test"]
	[D.content.somemap]
	life = 42
	st = 47
	leet = 1337
	`

	x := new(test)
	toml.Unmarshal([]byte(testStr), x)
	fmt.Printf("%#v\n", x)
	res, err := x.D.Content.ToTomlString()
	if err != nil {
		panic(err)
	}
	fmt.Printf("----\n%s\n", res)
}

&main.test{A:"this is A", B:10, C:3.14159, D:main.innerComplex{Type:"Something", Content:(*toml.Tree)(0xc000072960)}}
----
someList = ["this","is","a","test"]
stuff = "thing"
things = 3.14235

[somemap]
  leet = 1337
  life = 42
  st = 47

[pelletier]

Looking great! Patch is cleaner than I anticipated. Thank you for taking care of this!