Overwrite existing slices when unmarshaling

[SamWhited]

This brings the behavior inline with that of the standard library (eg.
encoding/json). This does change the behavior when the second slice is smaller (eg. before a new slice with 1 element would have overwritten just the first element of the existing slice, now you end up with a new slice). I think this is what most users will expect though (especially since it's what encoding/json and the like do).

Fixes #49
Fixes #99

Eg. the following now works (which would have produced a panic before):

package main

import (
    "fmt"
    "github.com/BurntSushi/toml"
)

func main() {
    blob := []byte(`
    test = ["aaa", "bbb"]

    [[table]]
    a = "test"
    `)

    type log struct {
        A string
        B string
    }

    type config struct {
        Table []log `toml:"table"`
        Test  []string
    }

    c := new(config)

    err := toml.Unmarshal(blob, &c)
    if err != nil {
        panic(err)
    }

    blob2 := []byte(`
    test = ["ccc"]

    [[table]]
    a = "test2"

    [[table]]
    a = "test3"
    `)

    err = toml.Unmarshal(blob2, &c)
    if err != nil {
        panic(err)
    }

    fmt.Printf("%+v\n", c)
}

Output:

&{Table:[{A:test2 B:} {A:test3 B:}] Test:[ccc]}

[cespare]

This was taken care of in #84.