We should support passing an array of string for the `hosts` options

I think we should either support or throw an exception when using an array of string for the `hosts` option:

```js
const searchClient = algoliasearch("", "", {
  hosts: ["test.com"]
});
```

instead of 

```js
const searchClient = algoliasearch("", "", {
  hosts: [{ url: "test.com/sitesearch" }]
});
```

Today it's creating a weird "undefined" domain.