feat: add extractChangelog to config

[kazupon]

In #388 , We can extract changelog for lerna-changelog, but when we use other changelog generating tool, we have to set updateChangelog = false.
In that case, when we execute shipjs release, we can't put changelog in GitHub Releases.

I've added new extractChangelog to config.

[eunjae-lee]

@kazupon Thanks for the contribution!

I've made some changes and I'd appreciate if you could review it 🙂

c2e6ddd, 5494eeb: just some clean-up of what I've done before inconsistently
6eedfc6: I've left comments there.

[kazupon]

@eunjae-lee
Thank you for your advice.
I've commented your comments.
I agree for your suggestion.

[eunjae-lee]

@kazupon Thanks for the review :)
It's merged and it will probably be released tomorrow 🎉

Feel free to open a new PR which adds you as a contributor.

yarn contributors:add kazupon code