Support axios `getUri`

https://github.com/axios/axios#config-defaults

As ["the goal is to support most/all features"](https://github.com/developit/redaxios/issues/14#issuecomment-615290952), supporting all [instance methods](https://github.com/axios/axios#instance-methods) also seems to be an important feature.

Currently, something like:

```js
let uri = axios.getUri(axiosConfig)
```

Results in:

```console
TypeError: axios.getUri is not a function
```

`getUri` method is specially useful when working with multiple instances, each one with a different `baseURL`.