feat(useAxios): axios instance support, close #273

[wheatjs]

Adds support for Axios instances as mentioned in issue #273.

Uses the following overrides so that it works with the existing API without breaking it.

export function useAxios<T = any>(url: string, config?: AxiosRequestConfig): UseAxiosReturn<T>
export function useAxios<T = any>(url: string, instance?: AxiosInstance): UseAxiosReturn<T>
export function useAxios<T = any>(url: string, config: AxiosRequestConfig, instance: AxiosInstance): UseAxiosReturn<T>

[antfu]

Is this a safe way to detect whether it's an axios instance?

[wheatjs]

I was going to use instanceof but it looks like you can't do that(axios/axios#737) so I used the way that was suggested here. It doesn't seem to have any overlap with the AxiosRequestConfig, so it works from my testing. But I guess the only real way to enforce it is with the typescript types for parameters. If you have any ideas on another way to do it let me know.

[antfu]

Alright, go with it then. Can you add a comment above that line mentioning about the issue you linked? Thanks.

[wheatjs]

Sure thing!