convert function

Add `convert` function

Converts specified keys into something else

```javascript
const a = {
  testa: 1,
  b: 2,
  c: 3,
};

convert(a, 'testa', 'a'); // => { a: 1, b: 2, c: 3 }
```

```javascript
const a = {
  testa: 1,
  testb: 2,
  c: 3,
};

convert(a, [
  'testa',
  'testb',
], [
  'a',
  'b',
]); // => { a: 1, b: 2, c: 3 }
```