Add reader mode

Created by: DavidVentura

Hi! This PR adds "Reader mode" to morph -- the readability.js file is taken straight out of Mozilla's repo here. The style and header are the bare minimum to get something usable, they could be improved.

I have absolutely no idea if the placement of my code makes any sense -- to me the 'readerMode' state should be stored in each tab, but that's likely incorrect

Attached, a video of behavior

https://user-images.githubusercontent.com/3650670/132724146-9555f134-5a55-45f7-bbf3-97a9ae66dfe9.mp4