Initial implementation
Support insets
Don't set style when element is outside viewport
Add a little to README
Math fix

Position is now a piecewise function.
Add more explanation to README
Bump to 0.1.1
Only add handlers once

I thought that duplicate handlers wouldn't be added but testing seems
to show I was wrong.
Bump to 0.1.2
Only call update if the scroll position changed
Use regular loops in event handlers.

For performance and stuff.
Remove forAxes method

I guess it was kinda dumb.
If no old position, consider value changed
Bump to 0.1.3