Support axios Config Defaults

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), [Config Defaults](https://github.com/axios/axios#config-defaults) also seems to be an important feature.

Currently, something like:

```js
instance.defaults.headers.get['Content-Type'] = 'application/json'`
```

Results in:

```console
Uncaught TypeError: instance.defaults.headers is undefined
```