Add support for passing a function to ky.extend()

[mfulton26]

callback function receives parent's default options to allow extended instance to refer to modify parent options

Closes #586

[sholladay]

Thanks for working on this, looking good.

Can you add a few more permutations to the tests? In particular, I want to make sure we cover:

1. What happens when the extend function ignores the parent options, i.e. is the prefixUrl still set if extend doesn't set it?
2. The behavior of setting nested options, such as hooks. Does setting a hook add to or replace the parent's hooks?

[mfulton26]

@sholladay I've added some more test cases. I believe I've captured what you were targeting.

[sholladay]

Such a simple change, yet I think this will be a popular feature and cleanly solve a lot of use cases. Thanks again!

[mfulton26]

Such a simple change, yet I think this will be a popular feature and cleanly solve a lot of use cases. Thanks again!

I agree. Thank you for helping me add this change. I look forward to using it in my own projects!

[joshgummersall]

Will this be published soon? Doesn't look like this landed in 1.4.0.

[sindresorhus]

https://github.com/sindresorhus/ky/releases/tag/v1.5.0

[joshgummersall]

Ah wow, thanks so much for getting to this to fast! Appreciate it 😄