Refactored the download_installer() function to accept full URL
(including file name) and file name as the second parameter.

* The second parameter (for download file name) must now be included
  in the URL as well as be supplied as the second argument.
* This enables the download_installer() function to handle download
  URL redirects where the file name is not exposed.
* This also enables the download_installer() function to handle
  downloads for temporary sessions (i.e. AWS files that use a
  download token).
Updated the settings.sh to make use of the new download URL key.

* All download URLs must include the file name now.
* Refactored the script so that application name, file name, and
  download URL are defined in the order mentioned here.
Refactored extension settings so paths are before URLs.

* This mimics the structure used for applications.
Added [Atom](https://atom.io) install.