Feature: Add loading progress bar

[orschiro]

On some pages such as this one OpenSpritz takes a very long time to actually start reading after I had set the wpm.

I then never know whether the site is broken or whether it just takes OpenSpritz a while to load the page.

Can we think of a visual indicator of the loading progress?

For example in the form of a simple text message:

Currently loading the text. Please wait...

[Miserlou]

This is definitely something I want in OpenSpritz 1.2. I've also emailed Diffbot and told them they need to turn on response caching, but no response. I hate slow things.