Support tags

Hello,

It would be great to support tags as the php version does
See: https://github.com/phaxio/phaxio-php/blob/32452c819e4553d64a6e52e9a7932d7a46e8a1d3/lib/Phaxio/Phaxio.php#L89

I tried hard-coding the tag as such and it works
```javascript
{
  'tag[name]': "value"
}
```

```javascript
But it would be better to support
{
  tags: {name: 'value'}
} 
```