Add create config option for custom axios instance

[sky-code]

I use axios from @nuxtjs/axios and I want to pass already created and configured axios instance in apisauce.
At this moment axios instance created from config parameters and then used by local instance variable, so this instance can't be changed.
src: https://github.com/infinitered/apisauce/blob/master/lib/apisauce.ts#L135
Can we add optional config parameter axiosInstance and then check if it set, use it, otherwise create new axios instance.

export const create = config => {
  let instance = config.axiosInstance
  if(!instance){
    instance = axios.create(combinedConfig)
  }
}

[robinheinze]

The is in 1.0.5