Fetch Instance

[nanotime]

Is your feature request related to a problem? Please describe.
The plugin need an inner full fetch instance to use internally on the different methods. This allows the users to use full vanilla fetch if they want and create inner instances with their own config.

Describe the solution you'd like
The method could be used like:

const instance = new HttpPlease({ ... });

instance.fetch(url, options); // using the full fetch API.

Describe alternatives you've considered
...

Additional context
This is primarily for better dev experience and sensibility.