[Feature Request]: Customize icon source from cargo config

[lexi-the-cute]

icon.png's path should be able to be specified from the cargo config otherwise it defaults to icon.png in the current working directory

[lexi-the-cute]

I did implement this in the PR I'm about to submit with the commit at master...lexi-the-cute:cargo-appimage:5b837717754b916b8bc5a8dd6b5a9b051d9352a2