feat: add pin property

[lunsdorf]

Implements issues #4, although I named the property pin instead of fixed since everything related is named like that.

This PR applies all changes from #131 as a patch together with an additional fix for the unit tests. I did it this way, because I deleted to fork I worked on years ago and couldn't figure out how to append to an existing PR with all sources deleted.

I did this because people keep asking if the changes could be integrated. I did not re-evaluate or actually test the changes I made when I originally wrote this. So somebody should definitly do that.

[janczizikow]

Hey @lunsdorf thanks for the PR! Looks like a nice thing to add! I'll try to have a closer look at it this week. Meanwhile I had a question that came to me while glancing at it initially:

What if the pin property changes once the Headroom has been already rendered? I think there might be some additional logic that should be added either in componentDidUpdate or getDerivedStateFromProps.

Also would be great if you could mention the prop in the docs (www/pages/index.md)

[lunsdorf]

Ok, I added the prop in the docs and also check for pin property changes in componentDidUpdate. I'm reusing the scroll handler function for this, as this is the one that uses requestAnimationFrame callback. The naming is a bit unfortunate however.

[janczizikow]

Is there any reason to put it in else if instead of if? Previous conditions check if props changed from disabled: true to disabled: false or the other way around. Those two cases are handled differently but it's unrelated to change of pin prop. So I think you could change it to a separate if instead of else if.

[lunsdorf]

I think I simply added it the the block I saw without any particular reason :) I changed it to a separate if.

[janczizikow]

LGTM 🎉