Set keep-alive/make it easier to set keep-alive flag in underlying fetch

[dlongley]

When this library is used in the browser we should be able to set keep-alive:

See: https://fetch.spec.whatwg.org/#requestinit

When creating a request for Fetch, ky takes options here: https://github.com/sindresorhus/ky/blob/v0.27.0/index.js#L184-L190 and passes them here: https://github.com/sindresorhus/ky/blob/v0.27.0/index.js#L232