feat(client): allow custom fetch impl

[drochetti]

This PR introduces the fetch function as a config option. It expects the function to follow the fetch signature and behavior, so node-fetch, isomorphic-fetch and similar can be use as a drop-in. When using HTTP clients such as Axios, one needs to write their own adapter function or use something like https://github.com/lifeomic/axios-fetch

[drochetti]

This addresses #70