Ability to provide custom fetch

[prichodko]

I am loving the simplicity of this module. What I also love is Next.js. However, those two unfortunately doesn't play well with each other.

Trying to call ky inside Next's getInitialProps throws that self is not defined, because during first request Next.js calls getInitialProps on the server.

One possible solution would be to accept a user's fetch (e.g. https://www.npmjs.com/package/isomorphic-unfetch).

I understand that this adds a complexity, but if you are willing to accept it, I can send a PR.

[sindresorhus]

Not sure I want to add an option for this, as it will enable users to supply a implementation that is not fully Fetch-compliant and then complain to me if something breaks. For example, unfetch is missing a lot and node-fetch is missing stuff too. I don't want to be forced to cater to those incomplete implementations.

[sindresorhus]

When #53 is fixed, you can just override the globals yourself with node-fetch.

[prichodko]

Haven't tried it yet, but I believe with the land of f107e7b we can close this.