Handle other key types in Unmarshal (#276)

Previously, this would fail with:

```
panic: reflect.Value.SetMapIndex: value of type string is not assignable to type toml.letter [recovered]
panic: reflect.Value.SetMapIndex: value of type string is not assignable to type toml.letter
```

Now this only panics when the key type cannot be converted from a
string.

marshal.go

@@ -615,7 +615,7 @@ func (d *Decoder) valueFromTree(mtype reflect.Type, tval *Tree) (reflect.Value,
 			if err != nil {
 				return mval, formatError(err, tval.GetPosition(key))
 			}
-			mval.SetMapIndex(reflect.ValueOf(key), mvalf)
+			mval.SetMapIndex(reflect.ValueOf(key).Convert(mtype.Key()), mvalf)
 		}
 	}
 	return mval, nil