This feature adds a horizontal bar under the top menu which tracks the vertical
scroll position. Such a feature can be useful to represent how much is left to
read on the current page more aesthetically.

As this is an optional feature, `enable_progressbar` must be set to `true` in
`_config.yml` to activate the functionality.

I am not the original author of this code. I just made it compatible with the
current version of the template at the time of this commit.
The original code was most likely authored by Pankaj Parashar in this
[post](https://css-tricks.com/reading-position-indicator/) made a few years
before the first inclusion in an `al-folio` site. Then, the code was adapted for
 compatibility with the template at Anthony  Plantanios' site. Finally, I did
the last updates to have the code fit the new conventions used in the project.