v0.4.2: Fix for custom buttons

Before this release you could define a custom icon for any existing button, e.g.

custom_icons:
  power: <svg_path_here>

The example given in the release notes is functional, you could set a custom icon for "hbo" and immediately add - hbo to any row or create a custom button named hbo, but the following would not work:

custom_icons:
  hbo: <svg_path>

custom_sources:
  hbomax:
    icon: hbo
   source: HBO Max

It would not work because the script would search for custom_icons["hbomax"], using the button name (hbomax), not the button icon (hbo).

Now it makes correct use of the icon parameter in the custom button.

Full Changelog: v0.4.1...v0.4.2