Add functions for fetching...

... abstracting the usage of `fetch`.

Examples:

```js
// in cerebro-tools

export const get = (url) => {
  return fetch(url)
    .then(response => response.json())
}

// in my plugin

import { get } from 'cerebro-tools'

get('from-some-api')
  .then(data => console.log('do something with', data))
```