Keep printliminated sections on screen and add keyboard shortcut Alt+Delete to remove hidden sections from the DOM

[odebroqueville]

First, I apologize with the formatting issues I've had that make it difficult to see the changes that I made to your code! I made these changes so that Printliminator would be compatible with Omnivore's web clipper, so that printliminated sections don't get saved to Omnivore.

I replaced event.which (which is deprecated) with event.key, and therefore also changed the keys object. I also added an entry for remove: to be able to remove the hidden sections from the DOM.

I also added the function removeHidden() which removes the hidden sections from the DOM.

When running grunt --force, I got some warnings I don't understand but nothing too bad as the changes are working.

Last but not least, I hope you'll consider transforming this useful extension to manifest v3 as Google will be deprecating mv2 come June.