Avoid use of globals where possible

[sholladay]

Fixes #64

This is similar to PR #69, however I've taken a different approach. Some of my goals include:

• No regex testing of URLs.
• Let fetch() handle URL resolution where possible.
• Guard against ReferenceErrors by not accessing globals when we don't need them.
• Simplify testing and mocking by treating global, window, and self as not mutually exclusive.

This should make things significantly easier in non-browser environments. In my testing on Node 10+, I was able to remove everything except for fetch and Response in test/_require.js and the test suite passed.

Previously, Ky would find the global object and then assume everything lived on that object, which makes things cumbersome in non-DOM / no-browser environments. Now, we lookup each global we need in each namespace. So for example, in Node you don't have to implement all of window anymore. If you assign a fetch implementation to window.fetch but nothing else, Ky will still be able to find URL in modern versions of Node where it lives on global and you won't have to assign it manually - it will just work.

Further, in all environments, globals are conditionally accessed to avoid unnecessary ReferenceErrors. Ky also no longer relies on the Headers class.

[sindresorhus]

I like this approach, except for not using Headers.

[sholladay]

@sindresorhus I think this is ready to go, unless you notice anything else.

Note that I included an unrelated fix for the Travis failures. It was still running Node 8.

[sindresorhus]

This is looking great! 👌

@sholladay Would you be interested in joining the project as a maintainer? You have done some really good PRs and you seem interested in the project.

[sholladay]

@sindresorhus Yes, would love to join. It's an awesome project. :)

[sindresorhus]

Yay. Welcome to the project! 🎉