Feature: Let user use custom top-panel icon

[matleborgne]

Hello,

It would be great to let user change the top-panel icon, defined in current installation by --icon <distro>.
The best would be to let user change path of the icon, or explain in the README how to find and replace the actual icon.

As a NixOS user, I'm stuck with Windows/Mac/Ubuntu or Arch icon.
For now, a workaround is to use dash-to-panel + arc-menu and use a custom icon in arc-menu, but it's not lightweight at all.

[vinceliuice]

Added support for nixos now

[matleborgne]

Thank's !

Just to thank you for your themes and icon packs, which are awesome, bravo :)
I close the issue then.